diff --git a/pkg/front_end/outline_extraction_testcases/.dart_tool/package_config.json b/pkg/front_end/outline_extraction_testcases/.dart_tool/package_config.json
new file mode 100644
index 00000000000..781616034bf
--- /dev/null
+++ b/pkg/front_end/outline_extraction_testcases/.dart_tool/package_config.json
@@ -0,0 +1,4 @@
+{
+  "configVersion": 2,
+  "packages": []
+}
\ No newline at end of file
diff --git a/pkg/front_end/outline_extraction_testcases/.packages b/pkg/front_end/outline_extraction_testcases/.packages
deleted file mode 100644
index 68325fa6d7e..00000000000
--- a/pkg/front_end/outline_extraction_testcases/.packages
+++ /dev/null
@@ -1 +0,0 @@
-# dummy
\ No newline at end of file
diff --git a/pkg/front_end/outline_extraction_testcases/factories/.dart_tool/package_config.json b/pkg/front_end/outline_extraction_testcases/factories/.dart_tool/package_config.json
new file mode 100644
index 00000000000..140964c143d
--- /dev/null
+++ b/pkg/front_end/outline_extraction_testcases/factories/.dart_tool/package_config.json
@@ -0,0 +1,9 @@
+{
+  "configVersion": 2,
+  "packages": [
+    {
+      "name": "foo",
+      "rootUri": ".."
+    }
+  ]
+}
\ No newline at end of file
diff --git a/pkg/front_end/outline_extraction_testcases/factories/.packages b/pkg/front_end/outline_extraction_testcases/factories/.packages
deleted file mode 100644
index 6607aecbc70..00000000000
--- a/pkg/front_end/outline_extraction_testcases/factories/.packages
+++ /dev/null
@@ -1 +0,0 @@
-foo:.
\ No newline at end of file
diff --git a/pkg/front_end/outline_extraction_testcases/named_import_with_export_and_named_constructor_and_class_type_parameter/.dart_tool/package_config.json b/pkg/front_end/outline_extraction_testcases/named_import_with_export_and_named_constructor_and_class_type_parameter/.dart_tool/package_config.json
new file mode 100644
index 00000000000..140964c143d
--- /dev/null
+++ b/pkg/front_end/outline_extraction_testcases/named_import_with_export_and_named_constructor_and_class_type_parameter/.dart_tool/package_config.json
@@ -0,0 +1,9 @@
+{
+  "configVersion": 2,
+  "packages": [
+    {
+      "name": "foo",
+      "rootUri": ".."
+    }
+  ]
+}
\ No newline at end of file
diff --git a/pkg/front_end/outline_extraction_testcases/named_import_with_export_and_named_constructor_and_class_type_parameter/.packages b/pkg/front_end/outline_extraction_testcases/named_import_with_export_and_named_constructor_and_class_type_parameter/.packages
deleted file mode 100644
index 6607aecbc70..00000000000
--- a/pkg/front_end/outline_extraction_testcases/named_import_with_export_and_named_constructor_and_class_type_parameter/.packages
+++ /dev/null
@@ -1 +0,0 @@
-foo:.
\ No newline at end of file
diff --git a/pkg/front_end/outline_extraction_testcases/split_import_export_part/.dart_tool/package_config.json b/pkg/front_end/outline_extraction_testcases/split_import_export_part/.dart_tool/package_config.json
new file mode 100644
index 00000000000..de99a55b009
--- /dev/null
+++ b/pkg/front_end/outline_extraction_testcases/split_import_export_part/.dart_tool/package_config.json
@@ -0,0 +1,9 @@
+{
+  "configVersion": 2,
+  "packages": [
+    {
+      "name": "foobar",
+      "rootUri": ".."
+    }
+  ]
+}
\ No newline at end of file
diff --git a/pkg/front_end/outline_extraction_testcases/split_import_export_part/.packages b/pkg/front_end/outline_extraction_testcases/split_import_export_part/.packages
deleted file mode 100644
index d19a7e08e3f..00000000000
--- a/pkg/front_end/outline_extraction_testcases/split_import_export_part/.packages
+++ /dev/null
@@ -1 +0,0 @@
-foobar:.
\ No newline at end of file
diff --git a/pkg/front_end/test/async_but_no_await_git_test.dart b/pkg/front_end/test/async_but_no_await_git_test.dart
index fb27bc24d74..0823d43084c 100644
--- a/pkg/front_end/test/async_but_no_await_git_test.dart
+++ b/pkg/front_end/test/async_but_no_await_git_test.dart
@@ -33,7 +33,7 @@ Future<void> main(List<String> args) async {
 
   Uri packageConfigUri = repoDir.resolve(".dart_tool/package_config.json");
   if (!new File.fromUri(packageConfigUri).existsSync()) {
-    throw "Couldn't find .packages";
+    throw "Couldn't find .dart_tool/package_config.json";
   }
   compilerOptions.packagesFileUri = packageConfigUri;
 
diff --git a/pkg/front_end/test/fasta/messages_suite.dart b/pkg/front_end/test/fasta/messages_suite.dart
index 3a90827898e..8c9862a1ea8 100644
--- a/pkg/front_end/test/fasta/messages_suite.dart
+++ b/pkg/front_end/test/fasta/messages_suite.dart
@@ -785,14 +785,13 @@ class Compile extends Step<Example?, Null, MessageTestSuite> {
     Uri output =
         suite.fileSystem.currentDirectory.resolve("$dir/main.dart.dill");
 
-    // Setup .packages if it (or package_config.json) doesn't exist.
-    Uri dotPackagesUri =
-        suite.fileSystem.currentDirectory.resolve("$dir/.packages");
-    Uri packageConfigJsonUri = suite.fileSystem.currentDirectory
+    // Setup .dart_tool/package_config.json if it doesn't exist.
+    Uri packageConfigUri = suite.fileSystem.currentDirectory
         .resolve("$dir/.dart_tool/package_config.json");
-    if (!await suite.fileSystem.entityForUri(dotPackagesUri).exists() &&
-        !await suite.fileSystem.entityForUri(packageConfigJsonUri).exists()) {
-      suite.fileSystem.entityForUri(dotPackagesUri).writeAsBytesSync([]);
+    if (!await suite.fileSystem.entityForUri(packageConfigUri).exists()) {
+      suite.fileSystem
+          .entityForUri(packageConfigUri)
+          .writeAsStringSync('{"configVersion": 2, "packages": []}');
     }
 
     print("Compiling $main");
@@ -805,7 +804,7 @@ class Compile extends Step<Example?, Null, MessageTestSuite> {
           ..explicitExperimentalFlags = example.experimentalFlags ?? {}
           ..target = new VmTarget(new TargetFlags())
           ..fileSystem = new HybridFileSystem(suite.fileSystem)
-          ..packagesFileUri = dotPackagesUri
+          ..packagesFileUri = packageConfigUri
           ..onDiagnostic = messages.add
           ..environmentDefines = const {}),
         main,
diff --git a/pkg/front_end/test/flutter_gallery_leak_tester.dart b/pkg/front_end/test/flutter_gallery_leak_tester.dart
index ff245b5befd..922bb318600 100644
--- a/pkg/front_end/test/flutter_gallery_leak_tester.dart
+++ b/pkg/front_end/test/flutter_gallery_leak_tester.dart
@@ -182,7 +182,7 @@ Future<void> main(List<String> args) async {
     "--output-dill",
     "$rootPath/flutter_server_tmp.dill",
     "--packages",
-    "$rootPath/gallery/.packages",
+    "$rootPath/gallery/.dart_tool/package_config.json",
     "-Ddart.vm.profile=false",
     "-Ddart.vm.product=false",
     "--enable-asserts",
diff --git a/pkg/front_end/test/id_testing/data/directory_testing/.dart_tool/package_config.json b/pkg/front_end/test/id_testing/data/directory_testing/.dart_tool/package_config.json
new file mode 100644
index 00000000000..27929acb345
--- /dev/null
+++ b/pkg/front_end/test/id_testing/data/directory_testing/.dart_tool/package_config.json
@@ -0,0 +1,9 @@
+{
+  "configVersion": 2,
+  "packages": [
+    {
+      "name": "foo",
+      "rootUri": "../lib"
+    }
+  ]
+}
diff --git a/pkg/front_end/test/incremental_suite.dart b/pkg/front_end/test/incremental_suite.dart
index e96910bd7cc..78f31b473dc 100644
--- a/pkg/front_end/test/incremental_suite.dart
+++ b/pkg/front_end/test/incremental_suite.dart
@@ -400,7 +400,7 @@ Future<Null> basicTest(YamlMap sourceFiles, String entryPoint,
       invalidateFilenames.remove(filename);
     }
     String source = sourceFiles[filename];
-    if (filename == ".packages") {
+    if (filename == ".dart_tool/package_config.json") {
       packagesUri = uri;
     }
     File file = new File.fromUri(uri);
@@ -477,7 +477,7 @@ Future<Map<String, List<int>>> createModules(
     Uri? packagesUri;
     for (String filename in module[moduleName].keys) {
       Uri uri = base.resolve(filename);
-      if (uri.pathSegments.last == ".packages") {
+      if (uri.pathSegments.last == "package_config.json") {
         packagesUri = uri;
       } else {
         moduleSources.add(uri);
@@ -683,9 +683,7 @@ class NewWorldTest {
       for (String filename in sourceFiles.keys) {
         String data = sourceFiles[filename] ?? "";
         Uri uri = base.resolve(filename);
-        if (filename == ".packages") {
-          packagesUri = uri;
-        } else if (filename == ".dart_tool/package_config.json") {
+        if (filename == ".dart_tool/package_config.json") {
           packagesUri = uri;
         }
         if (world["enableStringReplacement"] == true) {
@@ -693,8 +691,8 @@ class NewWorldTest {
         }
         fs.entityForUri(uri).writeAsStringSync(data);
       }
-      if (world["dotPackagesFile"] != null) {
-        packagesUri = base.resolve(world["dotPackagesFile"]);
+      if (world["packageConfigFile"] != null) {
+        packagesUri = base.resolve(world["packageConfigFile"]);
       }
 
       if (brandNewWorld) {
diff --git a/pkg/front_end/test/lint_suite.dart b/pkg/front_end/test/lint_suite.dart
index 2d919525051..009efff0191 100644
--- a/pkg/front_end/test/lint_suite.dart
+++ b/pkg/front_end/test/lint_suite.dart
@@ -167,23 +167,23 @@ class LintStep extends Step<LintTestDescription, LintTestDescription, Context> {
       description.cache.firstToken = scanner.tokenize();
       description.cache.lineStarts = scanner.lineStarts;
 
-      Uri dotPackages =
+      Uri packageConfig =
           description.uri.resolve(".dart_tool/package_config.json");
       while (true) {
-        if (new File.fromUri(dotPackages).existsSync()) {
+        if (new File.fromUri(packageConfig).existsSync()) {
           break;
         }
         // Stupid bailout.
-        if (dotPackages.pathSegments.length < Uri.base.pathSegments.length) {
+        if (packageConfig.pathSegments.length < Uri.base.pathSegments.length) {
           break;
         }
-        dotPackages =
-            dotPackages.resolve("../../.dart_tool/package_config.json");
+        packageConfig =
+            packageConfig.resolve("../../.dart_tool/package_config.json");
       }
 
-      File dotPackagesFile = new File.fromUri(dotPackages);
-      if (dotPackagesFile.existsSync()) {
-        description.cache.packages = await loadPackageConfigUri(dotPackages);
+      File packageConfigUri = new File.fromUri(packageConfig);
+      if (packageConfigUri.existsSync()) {
+        description.cache.packages = await loadPackageConfigUri(packageConfig);
       }
     }
 
diff --git a/pkg/front_end/test/outline_extractor_suite.dart b/pkg/front_end/test/outline_extractor_suite.dart
index 4548f0c4d3f..42adf8c80e5 100644
--- a/pkg/front_end/test/outline_extractor_suite.dart
+++ b/pkg/front_end/test/outline_extractor_suite.dart
@@ -100,7 +100,7 @@ class OutlineExtractorStep
   @override
   Future<Result<TestDescription>> run(
       TestDescription description, Context context) async {
-    Uri? packages = description.uri.resolve(".packages");
+    Uri? packages = description.uri.resolve(".dart_tool/package_config.json");
     if (!new File.fromUri(packages).existsSync()) {
       packages = null;
     }
@@ -141,7 +141,7 @@ class CompileAndCompareStep
   @override
   Future<Result<TestDescription>> run(
       TestDescription description, Context context) async {
-    Uri? packages = description.uri.resolve(".packages");
+    Uri? packages = description.uri.resolve(".dart_tool/package_config.json");
     if (!new File.fromUri(packages).existsSync()) {
       packages = null;
     }
diff --git a/pkg/front_end/testcases/incremental/changing_modules.yaml b/pkg/front_end/testcases/incremental/changing_modules.yaml
index 534a0f4c7bb..e6638ae50d1 100644
--- a/pkg/front_end/testcases/incremental/changing_modules.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules.yaml
@@ -22,8 +22,16 @@ modules:
       b() {
         a();
       }
-    example_0.1.0/.packages: |
-      example:.
+    example_0.1.0/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "example",
+            "rootUri": ".."
+          }
+        ]
+      }
   example_0.1.1:
     example_0.1.1/b.dart: |
       // @dart=2.9
@@ -31,8 +39,16 @@ modules:
         print("Hello from v0.1.1");
       }
       bool example011 = true;
-    example_0.1.1/.packages: |
-      example:.
+    example_0.1.1/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "example",
+            "rootUri": ".."
+          }
+        ]
+      }
   foo_1:
     foo_1/foo.dart: |
       // @dart=2.9
@@ -42,9 +58,20 @@ modules:
         b();
       }
       bool foo1 = true;
-    foo_1/.packages: |
-      foo:.
-      example:../example_0.1.0
+    foo_1/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "foo",
+            "rootUri": ".."
+          },
+          {
+            "name": "example",
+            "rootUri": "../../example_0.1.0"
+          }
+        ]
+      }
   foo_2:
     foo_2/foo.dart: |
       // @dart=2.9
@@ -68,9 +95,20 @@ modules:
       baz() {
         print("hello from baz");
       }
-    foo_2/.packages: |
-      foo:.
-      example:../example_0.1.1
+    foo_2/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "foo",
+            "rootUri": ".."
+          },
+          {
+            "name": "example",
+            "rootUri": "../../example_0.1.1"
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -82,7 +120,16 @@ worlds:
           print("hello");
           b();
         }
-      .packages: example:example_0.1.0
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../example_0.1.0"
+            }
+          ]
+        }
     modules:
       - example_0.1.0
     expectedLibraryCount: 3
@@ -108,9 +155,20 @@ worlds:
           print("hello");
           foo();
         }
-      .packages: |
-        example:example_0.1.0
-        foo:foo_1
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../example_0.1.0"
+            },
+            {
+              "name": "foo",
+              "rootUri": "../foo_1"
+            }
+          ]
+        }
     modules:
       - example_0.1.0
       - foo_1
@@ -139,9 +197,20 @@ worlds:
           print("hello");
           foo();
         }
-      .packages: |
-        example:example_0.1.1
-        foo:foo_2
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../example_0.1.1"
+            },
+            {
+              "name": "foo",
+              "rootUri": "../foo_2"
+            }
+          ]
+        }
     modules:
       - example_0.1.1
       - foo_2
diff --git a/pkg/front_end/testcases/incremental/changing_modules_10.yaml b/pkg/front_end/testcases/incremental/changing_modules_10.yaml
index 1cdaafd10b1..e81be3815af 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_10.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_10.yaml
@@ -18,8 +18,16 @@ modules:
     module/b.dart: |
       // @dart=2.9
       class B { }
-    module/.packages: |
-      module:.
+    module/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "module",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -28,8 +36,16 @@ worlds:
         // @dart=2.9
         import 'package:module/a.dart';
         class Foo extends A {}
-      .packages: |
-        module:module
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "module",
+              "rootUri": "../module"
+            }
+          ]
+        }
     modules:
       - module
     expectedLibraryCount: 3
diff --git a/pkg/front_end/testcases/incremental/changing_modules_11.yaml b/pkg/front_end/testcases/incremental/changing_modules_11.yaml
index aedd5d0f9db..e433d986ff6 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_11.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_11.yaml
@@ -18,8 +18,16 @@ modules:
     module/b.dart: |
       // @dart=2.9
       class B { }
-    module/.packages: |
-      module:.
+    module/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "module",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -28,8 +36,16 @@ worlds:
         // @dart=2.9
         import 'package:module/a.dart';
         class Foo extends A {}
-      .packages: |
-        module:module
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "module",
+              "rootUri": "../module"
+            }
+          ]
+        }
     modules:
       - module
     expectedLibraryCount: 3
diff --git a/pkg/front_end/testcases/incremental/changing_modules_12.yaml b/pkg/front_end/testcases/incremental/changing_modules_12.yaml
index c8f1caea8cc..082cc0ab881 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_12.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_12.yaml
@@ -24,8 +24,16 @@ modules:
       // @dart=2.9
       import 'package:module/c.dart';
       class D { }
-    module/.packages: |
-      module:.
+    module/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "module",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -33,8 +41,16 @@ worlds:
       main.dart: |
         // @dart=2.9
         class Foo {}
-      .packages: |
-        module:module
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "module",
+              "rootUri": "../module"
+            }
+          ]
+        }
     modules:
       - module
     expectedLibraryCount: 1
diff --git a/pkg/front_end/testcases/incremental/changing_modules_13.yaml b/pkg/front_end/testcases/incremental/changing_modules_13.yaml
index 5d042e91c13..d92b7ca6e74 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_13.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_13.yaml
@@ -14,8 +14,16 @@ modules:
       class B { }
       class C { }
       class AB = A with B;
-    module/.packages: |
-      module:.
+    module/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "module",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -24,8 +32,16 @@ worlds:
         // @dart=2.9
         import "package:module/a.dart";
         class ABC = AB with C;
-      .packages: |
-        module:module
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "module",
+              "rootUri": "../module"
+            }
+          ]
+        }
     modules:
       - module
     expectedLibraryCount: 2
diff --git a/pkg/front_end/testcases/incremental/changing_modules_14.yaml b/pkg/front_end/testcases/incremental/changing_modules_14.yaml
index 641accff220..77d4395c8ba 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_14.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_14.yaml
@@ -12,16 +12,37 @@ target: DDC
 modules:
   moduleC:
     moduleC/lib.dart: |
+      // @dart=2.9
       const constC = ['value_c'];
-    moduleC/.packages: |
-      moduleC:.
+    moduleC/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleC",
+            "rootUri": ".."
+          }
+        ]
+      }
   moduleB:
     moduleB/lib.dart: |
+      // @dart=2.9
       import "package:moduleC/lib.dart";
       const constB = ['value_b', constC];
-    moduleB/.packages: |
-      moduleB:.
-      moduleC:../moduleC
+    moduleB/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleB",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleC",
+            "rootUri": "../../moduleC"
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -33,9 +54,20 @@ worlds:
         const constLib = constFromLib;
       lib.dart: |
         const constFromLib = 42;
-      .packages: |
-        moduleB:moduleB
-        moduleC:moduleC
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "moduleB",
+              "rootUri": "../moduleB"
+            },
+            {
+              "name": "moduleC",
+              "rootUri": "../moduleC"
+            }
+          ]
+        }
     modules:
       - moduleB
       - moduleC
diff --git a/pkg/front_end/testcases/incremental/changing_modules_15.yaml b/pkg/front_end/testcases/incremental/changing_modules_15.yaml
index d0753b84d79..f1c41021cd1 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_15.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_15.yaml
@@ -12,16 +12,37 @@ target: DDC
 modules:
   moduleC:
     moduleC/lib.dart: |
+      // @dart=2.9
       const constC = true;
-    moduleC/.packages: |
-      moduleC:.
+    moduleC/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleC",
+            "rootUri": ".."
+          }
+        ]
+      }
   moduleB:
     moduleB/lib.dart: |
+      // @dart=2.9
       import "package:moduleC/lib.dart";
       const constB = false || constC;
-    moduleB/.packages: |
-      moduleB:.
-      moduleC:../moduleC
+    moduleB/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleB",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleC",
+            "rootUri": "../../moduleC"
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -29,9 +50,20 @@ worlds:
       main.dart: |
         import 'package:moduleB/lib.dart';
         const constA = true && constB;
-      .packages: |
-        moduleB:moduleB
-        moduleC:moduleC
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "moduleB",
+              "rootUri": "../moduleB"
+            },
+            {
+              "name": "moduleC",
+              "rootUri": "../moduleC"
+            }
+          ]
+        }
     modules:
       - moduleB
       - moduleC
diff --git a/pkg/front_end/testcases/incremental/changing_modules_2.yaml b/pkg/front_end/testcases/incremental/changing_modules_2.yaml
index b8db340e24c..a73afd8c3b2 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_2.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_2.yaml
@@ -12,22 +12,50 @@ modules:
       // @dart=2.9
       a() {}
       var example = true;
-    example_1/.packages: |
-      example:.
+    example_1/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "example",
+            "rootUri": ".."
+          }
+        ]
+      }
   example_2:
     example_2/a.dart: |
       // @dart=2.9
       a() {}
       var example = true;
-    example_2/.packages: |
-      example:.
+    example_2/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "example",
+            "rootUri": ".."
+          }
+        ]
+      }
   foo:
     foo/foo.dart: |
+      // @dart=2.9
       export "package:example/a.dart";
       var foo = true;
-    foo/.packages: |
-      foo:.
-      example:../example_1
+    foo/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "foo",
+            "rootUri": ".."
+          },
+          {
+            "name": "example",
+            "rootUri": "../../example_1"
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -38,9 +66,20 @@ worlds:
           print("hello");
           a();
         }
-      .packages: |
-        foo:foo
-        example:example_1
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": "../foo"
+            },
+            {
+              "name": "example",
+              "rootUri": "../../example_1"
+            }
+          ]
+        }
     modules:
       - foo
       - example_1
@@ -60,9 +99,20 @@ worlds:
     worldType: updated
     expectInitializeFromDill: false
     sources:
-      .packages: |
-        foo:foo
-        example:example_2
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": "../foo"
+            },
+            {
+              "name": "example",
+              "rootUri": "../../example_2"
+            }
+          ]
+        }
     modules:
       - foo
       - example_2
diff --git a/pkg/front_end/testcases/incremental/changing_modules_3.yaml b/pkg/front_end/testcases/incremental/changing_modules_3.yaml
index fdfb2a6c4ae..b8e864b88f3 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_3.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_3.yaml
@@ -11,14 +11,30 @@ modules:
     foo/foo.dart: |
       // @dart=2.9
       var foo = true;
-    foo/.packages: |
-      foo:.
+    foo/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "foo",
+            "rootUri": ".."
+          }
+        ]
+      }
   foo2:
     foo2/foo.dart: |
       // @dart=2.9
       var foo2 = true;
-    foo2/.packages: |
-      foo:.
+    foo2/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "foo",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -30,8 +46,16 @@ worlds:
         main() {
           print(foo);
         }
-      .packages: |
-        foo:foo
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": "../foo"
+            }
+          ]
+        }
     modules:
       - foo
     expectedLibraryCount: 2
@@ -55,8 +79,16 @@ worlds:
         main() {
           print(foo2);
         }
-      .packages: |
-        foo:foo2
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": "../foo2"
+            }
+          ]
+        }
     modules:
       - foo2
     expectedLibraryCount: 2
diff --git a/pkg/front_end/testcases/incremental/changing_modules_4.yaml b/pkg/front_end/testcases/incremental/changing_modules_4.yaml
index 98ba2fa5a27..3e155a280b0 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_4.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_4.yaml
@@ -19,10 +19,24 @@ modules:
       class A2 extends A3 {
         int bar = 42;
       }
-    moduleB/.packages: |
-      moduleB:.
-      moduleC:../moduleC
-      moduleD:../moduleD
+    moduleB/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleB",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleC",
+            "rootUri": "../../moduleC"
+          },
+          {
+            "name": "moduleD",
+            "rootUri": "../../moduleD"
+          }
+        ]
+      }
   moduleC:
     moduleC/c.dart: |
       // @dart=2.9
@@ -34,15 +48,34 @@ modules:
       class A3 {
         int foo = 42;
       }
-    moduleC/.packages: |
-      moduleC:.
-      moduleD:../moduleD
+    moduleC/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleC",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleD",
+            "rootUri": "../../moduleD"
+          }
+        ]
+      }
   moduleD:
     moduleD/d.dart: |
       // @dart=2.9
       String baz3 = "baz3";
-    moduleD/.packages: |
-      moduleD:.
+    moduleD/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleD",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: a.dart
     fromComponent: true
@@ -53,10 +86,24 @@ worlds:
 
         String foo = baz;
         var x = mya2.bar;
-      .packages: |
-        moduleB:moduleB
-        moduleC:moduleC
-        moduleD:moduleD
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "moduleB",
+              "rootUri": "../moduleB"
+            },
+            {
+              "name": "moduleC",
+              "rootUri": "../moduleC"
+            },
+            {
+              "name": "moduleD",
+              "rootUri": "../moduleD"
+            }
+          ]
+        }
     modules:
       - moduleB
       - moduleC
diff --git a/pkg/front_end/testcases/incremental/changing_modules_5.yaml b/pkg/front_end/testcases/incremental/changing_modules_5.yaml
index b044f038fe6..4cd4a5f52af 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_5.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_5.yaml
@@ -20,10 +20,24 @@ modules:
       class A2 extends A3 {
         int bar = 42;
       }
-    moduleB/.packages: |
-      moduleB:.
-      moduleC:../moduleC
-      moduleD:../moduleD
+    moduleB/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleB",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleC",
+            "rootUri": "../../moduleC"
+          },
+          {
+            "name": "moduleD",
+            "rootUri": "../../moduleD"
+          }
+        ]
+      }
   moduleC:
     moduleC/c.dart: |
       // @dart=2.9
@@ -35,15 +49,34 @@ modules:
       class A3 {
         int foo = 42;
       }
-    moduleC/.packages: |
-      moduleC:.
-      moduleD:../moduleD
+    moduleC/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleC",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleD",
+            "rootUri": "../../moduleD"
+          }
+        ]
+      }
   moduleD:
     moduleD/d.dart: |
       // @dart=2.9
       String baz3 = "baz3";
-    moduleD/.packages: |
-      moduleD:.
+    moduleD/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleD",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: a.dart
     fromComponent: true
@@ -54,10 +87,24 @@ worlds:
 
         var foo = bVar;
         var foo2 = bVarFromC;
-      .packages: |
-        moduleB:moduleB
-        moduleC:moduleC
-        moduleD:moduleD
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "moduleB",
+              "rootUri": "../moduleB"
+            },
+            {
+              "name": "moduleC",
+              "rootUri": "../moduleC"
+            },
+            {
+              "name": "moduleD",
+              "rootUri": "../moduleD"
+            }
+          ]
+        }
     modules:
       - moduleB
       - moduleC
diff --git a/pkg/front_end/testcases/incremental/changing_modules_6.yaml b/pkg/front_end/testcases/incremental/changing_modules_6.yaml
index c66ca3b9ca3..ee9094bb6dd 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_6.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_6.yaml
@@ -20,10 +20,24 @@ modules:
       class A2 extends A3 {
         int bar = 42;
       }
-    moduleB/.packages: |
-      moduleB:.
-      moduleC:../moduleC
-      moduleD:../moduleD
+    moduleB/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleB",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleC",
+            "rootUri": "../../moduleC"
+          },
+          {
+            "name": "moduleD",
+            "rootUri": "../../moduleD"
+          }
+        ]
+      }
   moduleC:
     moduleC/c.dart: |
       // @dart=2.9
@@ -35,15 +49,34 @@ modules:
       class A3 {
         int foo = 42;
       }
-    moduleC/.packages: |
-      moduleC:.
-      moduleD:../moduleD
+    moduleC/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleC",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleD",
+            "rootUri": "../../moduleD"
+          }
+        ]
+      }
   moduleD:
     moduleD/d.dart: |
       // @dart=2.9
       String baz3 = "baz3";
-    moduleD/.packages: |
-      moduleD:.
+    moduleD/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleD",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: a.dart
     fromComponent: true
@@ -58,10 +91,24 @@ worlds:
         class A1 extends A2 {
           String fooMethod() => "42!";
         }
-      .packages: |
-        moduleB:moduleB
-        moduleC:moduleC
-        moduleD:moduleD
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "moduleB",
+              "rootUri": "../moduleB"
+            },
+            {
+              "name": "moduleC",
+              "rootUri": "../moduleC"
+            },
+            {
+              "name": "moduleD",
+              "rootUri": "../moduleD"
+            }
+          ]
+        }
     modules:
       - moduleB
       - moduleC
diff --git a/pkg/front_end/testcases/incremental/changing_modules_7.yaml b/pkg/front_end/testcases/incremental/changing_modules_7.yaml
index f718f140416..28632d3b539 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_7.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_7.yaml
@@ -36,9 +36,18 @@ modules:
         String str() {}
       }
     module/lib4.dart: |
+      // @dart=2.9
       class Class4 {}
-    module/.packages: |
-      module:.
+    module/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "module",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: compileme.dart
     fromComponent: true
@@ -60,8 +69,16 @@ worlds:
             return class1;
           }
         }
-      .packages: |
-        module:module
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "module",
+              "rootUri": "../module"
+            }
+          ]
+        }
     modules:
       - module
     expectedLibraryCount: 5
@@ -105,8 +122,16 @@ worlds:
             return class1;
           }
         }
-      .packages: |
-        module:module
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "module",
+              "rootUri": "../module"
+            }
+          ]
+        }
     modules:
       - module
     expectedLibraryCount: 5
diff --git a/pkg/front_end/testcases/incremental/changing_modules_8.yaml b/pkg/front_end/testcases/incremental/changing_modules_8.yaml
index 42de9b44b32..118080cfdef 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_8.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_8.yaml
@@ -31,8 +31,16 @@ modules:
       class XIdentityHashSet implements XLinkedHashSet {
         XIdentityHashSet();
       }
-    module/.packages: |
-      module:.
+    module/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "module",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: compileme.dart
     fromComponent: true
@@ -44,8 +52,16 @@ worlds:
         main() {
           XSet history = new XSet.identity();
         }
-      .packages: |
-        module:module
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "module",
+              "rootUri": "../module"
+            }
+          ]
+        }
     modules:
       - module
     expectedLibraryCount: 4
diff --git a/pkg/front_end/testcases/incremental/changing_modules_9.yaml b/pkg/front_end/testcases/incremental/changing_modules_9.yaml
index 85ef80a270c..e267a819ba6 100644
--- a/pkg/front_end/testcases/incremental/changing_modules_9.yaml
+++ b/pkg/front_end/testcases/incremental/changing_modules_9.yaml
@@ -14,17 +14,36 @@ modules:
       export 'package:moduleB/b.dart';
 
       class A { }
-    moduleA/.packages: |
-      moduleA:.
-      moduleB:../moduleB
+    moduleA/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleA",
+            "rootUri": ".."
+          },
+          {
+            "name": "moduleB",
+            "rootUri": "../../moduleB"
+          }
+        ]
+      }
   moduleB:
     moduleB/b.dart: |
       // @dart=2.9
       class B {
         int foo = 42;
       }
-    moduleB/.packages: |
-      moduleB:.
+    moduleB/.dart_tool/package_config.json: |
+      {
+        "configVersion": 2,
+        "packages": [
+          {
+            "name": "moduleB",
+            "rootUri": ".."
+          }
+        ]
+      }
 worlds:
   - entry: main.dart
     fromComponent: true
@@ -36,9 +55,20 @@ worlds:
         main() {
           A a = new A();
         }
-      .packages: |
-        moduleA:moduleA
-        moduleB:moduleB
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "moduleA",
+              "rootUri": "../moduleA"
+            },
+            {
+              "name": "moduleB",
+              "rootUri": "../moduleB"
+            }
+          ]
+        }
     modules:
       - moduleA
       - moduleB
diff --git a/pkg/front_end/testcases/incremental/cleans_up_uritosource_unreferenced_package_library.yaml b/pkg/front_end/testcases/incremental/cleans_up_uritosource_unreferenced_package_library.yaml
index 4e07d258de1..ea47771954d 100644
--- a/pkg/front_end/testcases/incremental/cleans_up_uritosource_unreferenced_package_library.yaml
+++ b/pkg/front_end/testcases/incremental/cleans_up_uritosource_unreferenced_package_library.yaml
@@ -2,16 +2,25 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE.md file.
 
-# Compile an application with a package. Update the world so that the .packages
-# no longer reference the package (and the source no longe ruse it) and
-# recompile. The now no longer referenced package library should also have been
-# removed from the uri to sources.
+# Compile an application with a package. Update the world so that the
+# .dart_tool/package_config.json no longer reference the package (and the source
+# no longer use it) and recompile. The now no longer referenced package library
+# should also have been removed from the uri to sources.
 
 type: newworld
 worlds:
   - entry: main.dart
     sources:
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         import "package:example/b.dart";
@@ -24,10 +33,14 @@ worlds:
     worldType: updated
     invalidate:
       - main.dart
-      - .packages
+      - .dart_tool/package_config.json
     expectInitializeFromDill: false
     sources:
-      .packages: 
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": []
+        }
       main.dart: |
         // @dart=2.9
         main() {}
diff --git a/pkg/front_end/testcases/incremental/disappearing_package.yaml b/pkg/front_end/testcases/incremental/disappearing_package.yaml
index fe8d1575c53..5349d99e6a4 100644
--- a/pkg/front_end/testcases/incremental/disappearing_package.yaml
+++ b/pkg/front_end/testcases/incremental/disappearing_package.yaml
@@ -3,8 +3,8 @@
 # BSD-style license that can be found in the LICENSE.md file.
 
 # Compile an application with a package and use it.
-# Then remove the package from .packages and remove any use of it.
-# The package is still included in the dill file we initialize from,
+# Then remove the package from .dart_tool/package_config.json and remove any use
+# of it. The package is still included in the dill file we initialize from,
 # but shouldn't cause trouble, nor be included in the output.
 
 type: newworld
@@ -23,7 +23,16 @@ worlds:
         b() {
           print("b");
         }
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
     expectedLibraryCount: 2
   - entry: main.dart
     # TODO(jensj): For now we don't initialize from dill when a package was
@@ -37,5 +46,9 @@ worlds:
         main() {
           print("hello");
         }
-      .packages:
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": []
+        }
     expectedLibraryCount: 1
diff --git a/pkg/front_end/testcases/incremental/entry_not_package_url_main.yaml b/pkg/front_end/testcases/incremental/entry_not_package_url_main.yaml
index 4260828facd..1f881a70182 100644
--- a/pkg/front_end/testcases/incremental/entry_not_package_url_main.yaml
+++ b/pkg/front_end/testcases/incremental/entry_not_package_url_main.yaml
@@ -13,7 +13,16 @@ worlds:
         // @dart=2.9
         main() {
         }
-      .packages: untitled:/
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "untitled",
+              "rootUri": "/"
+            }
+          ]
+        }
     expectedLibraryCount: 1
     errors: false
     warnings: false
diff --git a/pkg/front_end/testcases/incremental/entry_not_package_url_main_with_errors.yaml b/pkg/front_end/testcases/incremental/entry_not_package_url_main_with_errors.yaml
index e7b80f2e1fb..4c8beb6bfdd 100644
--- a/pkg/front_end/testcases/incremental/entry_not_package_url_main_with_errors.yaml
+++ b/pkg/front_end/testcases/incremental/entry_not_package_url_main_with_errors.yaml
@@ -15,7 +15,16 @@ worlds:
         main() {
           asdf;
         }
-      .packages: untitled:/
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "untitled",
+              "rootUri": "/"
+            }
+          ]
+        }
     expectedLibraryCount: 1
     errors: true
     warnings: false
diff --git a/pkg/front_end/testcases/incremental/entry_not_package_url_no_main.yaml b/pkg/front_end/testcases/incremental/entry_not_package_url_no_main.yaml
index 72eac403f6a..a070cbbea1d 100644
--- a/pkg/front_end/testcases/incremental/entry_not_package_url_no_main.yaml
+++ b/pkg/front_end/testcases/incremental/entry_not_package_url_no_main.yaml
@@ -12,7 +12,16 @@ worlds:
       main.dart: |
         // @dart=2.9
         notMain() {}
-      .packages: untitled:/
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "untitled",
+              "rootUri": "/"
+            }
+          ]
+        }
     expectedLibraryCount: 1
     errors: false
     warnings: false
diff --git a/pkg/front_end/testcases/incremental/flutter_widget_transform.yaml b/pkg/front_end/testcases/incremental/flutter_widget_transform.yaml
index 0072fcb0e8d..01cc15f6db0 100644
--- a/pkg/front_end/testcases/incremental/flutter_widget_transform.yaml
+++ b/pkg/front_end/testcases/incremental/flutter_widget_transform.yaml
@@ -51,8 +51,16 @@ worlds:
           /// Optional locations of the parameters of the member at this location.
           final List<_Location> parameterLocations;
         }
-      .packages: |
-        flutter:flutter/lib
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "flutter",
+              "rootUri": "../flutter/lib"
+            }
+          ]
+        }
     expectedLibraryCount: 3
   - entry: main.dart
     worldType: updated
diff --git a/pkg/front_end/testcases/incremental/incremental_serialization_1.yaml b/pkg/front_end/testcases/incremental/incremental_serialization_1.yaml
index fa1166f1442..3e639115357 100644
--- a/pkg/front_end/testcases/incremental/incremental_serialization_1.yaml
+++ b/pkg/front_end/testcases/incremental/incremental_serialization_1.yaml
@@ -34,9 +34,20 @@ worlds:
         lib3() {
           return lib1();
         }
-      .packages: |
-        package1:package1
-        package2:package2
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "package1",
+              "rootUri": "../package1"
+            },
+            {
+              "name": "package2",
+              "rootUri": "../package2"
+            }
+          ]
+        }
     expectedLibraryCount: 4
     incrementalSerializationDoesWork: true
   - entry: main.dart
diff --git a/pkg/front_end/testcases/incremental/incremental_serialization_2.yaml b/pkg/front_end/testcases/incremental/incremental_serialization_2.yaml
index 997b7378f22..15fb17a1982 100644
--- a/pkg/front_end/testcases/incremental/incremental_serialization_2.yaml
+++ b/pkg/front_end/testcases/incremental/incremental_serialization_2.yaml
@@ -36,8 +36,16 @@ worlds:
         lib3() {
           return lib1();
         }
-      .packages: |
-        package1:package1
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "package1",
+              "rootUri": "../package1"
+            }
+          ]
+        }
     expectedLibraryCount: 4
     incrementalSerializationDoesWork: true
   - entry: main.dart
diff --git a/pkg/front_end/testcases/incremental/incremental_serialization_3.yaml b/pkg/front_end/testcases/incremental/incremental_serialization_3.yaml
index 775b3229051..e3664d70d4e 100644
--- a/pkg/front_end/testcases/incremental/incremental_serialization_3.yaml
+++ b/pkg/front_end/testcases/incremental/incremental_serialization_3.yaml
@@ -46,10 +46,24 @@ worlds:
         lib3() {
           print("lib3");
         }
-      .packages: |
-        package1:package1
-        package2:package2
-        package3:package3
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "package1",
+              "rootUri": "../package1"
+            },
+            {
+              "name": "package2",
+              "rootUri": "../package2"
+            },
+            {
+              "name": "package3",
+              "rootUri": "../package3"
+            }
+          ]
+        }
     expectedLibraryCount: 5
     incrementalSerializationDoesWork: true
   - entry: main.dart
@@ -89,9 +103,23 @@ worlds:
         lib3() {
           print("lib3");
         }
-      .packages: |
-        package1:package1
-        package2:package2
-        package3:package3
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "package1",
+              "rootUri": "../package1"
+            },
+            {
+              "name": "package2",
+              "rootUri": "../package2"
+            },
+            {
+              "name": "package3",
+              "rootUri": "../package3"
+            }
+          ]
+        }
     expectedLibraryCount: 5
     incrementalSerializationDoesWork: true
diff --git a/pkg/front_end/testcases/incremental/incremental_serialization_4.yaml b/pkg/front_end/testcases/incremental/incremental_serialization_4.yaml
index a9c75d538dd..144fc2c6f0f 100644
--- a/pkg/front_end/testcases/incremental/incremental_serialization_4.yaml
+++ b/pkg/front_end/testcases/incremental/incremental_serialization_4.yaml
@@ -45,9 +45,20 @@ worlds:
           p1();
           print("Package 2");
         }
-      .packages: |
-        package1:package1
-        package2:package2
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "package1",
+              "rootUri": "../package1"
+            },
+            {
+              "name": "package2",
+              "rootUri": "../package2"
+            }
+          ]
+        }
     expectedLibraryCount: 3
     incrementalSerializationDoesWork: true
   - entry: main.dart
@@ -86,8 +97,19 @@ worlds:
           p1();
           print("Package 2");
         }
-      .packages: |
-        package1:package1v2
-        package2:package2
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "package1",
+              "rootUri": "../package1v2"
+            },
+            {
+              "name": "package2",
+              "rootUri": "../package2"
+            }
+          ]
+        }
     expectedLibraryCount: 3
     incrementalSerializationDoesWork: true
diff --git a/pkg/front_end/testcases/incremental/initialize_with_unused_package_then_use_type.yaml b/pkg/front_end/testcases/incremental/initialize_with_unused_package_then_use_type.yaml
index 6460d792820..dd9cc980a82 100644
--- a/pkg/front_end/testcases/incremental/initialize_with_unused_package_then_use_type.yaml
+++ b/pkg/front_end/testcases/incremental/initialize_with_unused_package_then_use_type.yaml
@@ -27,7 +27,16 @@ worlds:
             print("Foo!");
           }
         }
-      .packages: mypackage:mypackage
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "mypackage",
+              "rootUri": "../mypackage"
+            }
+          ]
+        }
     expectedLibraryCount: 2
   - entry: main.dart
     invalidate:
@@ -45,7 +54,16 @@ worlds:
             print("Foo!");
           }
         }
-      .packages: mypackage:mypackage
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "mypackage",
+              "rootUri": "../mypackage"
+            }
+          ]
+        }
     expectedLibraryCount: 1
     expectInitializeFromDill: true
   - entry: main.dart
diff --git a/pkg/front_end/testcases/incremental/invalidate_package_part.yaml b/pkg/front_end/testcases/incremental/invalidate_package_part.yaml
index 7fee6d061aa..224d6154df2 100644
--- a/pkg/front_end/testcases/incremental/invalidate_package_part.yaml
+++ b/pkg/front_end/testcases/incremental/invalidate_package_part.yaml
@@ -22,7 +22,16 @@ worlds:
         b() {
           print("b1");
         }
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
     expectedLibraryCount: 1
   - entry: "package:example/main.dart"
     invalidate:
@@ -43,7 +52,16 @@ worlds:
         b() {
           print("b2");
         }
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
     expectedLibraryCount: 1
   - entry: "package:example/main.dart"
     invalidate:
@@ -64,5 +82,14 @@ worlds:
         b() {
           print("b3");
         }
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
     expectedLibraryCount: 1
diff --git a/pkg/front_end/testcases/incremental/invalidate_package_part_2.yaml b/pkg/front_end/testcases/incremental/invalidate_package_part_2.yaml
index 2e6331140fe..c8635e8574b 100644
--- a/pkg/front_end/testcases/incremental/invalidate_package_part_2.yaml
+++ b/pkg/front_end/testcases/incremental/invalidate_package_part_2.yaml
@@ -21,7 +21,16 @@ worlds:
         b() {
           print("b1");
         }
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
     expectedLibraryCount: 1
   - entry: "package:example/main.dart"
     worldType: updated
@@ -37,7 +46,16 @@ worlds:
         b() {
           print("b2");
         }
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
     expectedLibraryCount: 1
   - entry: "package:example/main.dart"
     worldType: updated
@@ -53,5 +71,14 @@ worlds:
         b() {
           print("b3");
         }
-      .packages: example:pkg/example
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../pkg/example"
+            }
+          ]
+        }
     expectedLibraryCount: 1
diff --git a/pkg/front_end/testcases/incremental/invalidate_package_part_as_file.yaml b/pkg/front_end/testcases/incremental/invalidate_package_part_as_file.yaml
index 4e2cd4da10e..b6fc99f4ad4 100644
--- a/pkg/front_end/testcases/incremental/invalidate_package_part_as_file.yaml
+++ b/pkg/front_end/testcases/incremental/invalidate_package_part_as_file.yaml
@@ -22,4 +22,13 @@ sources:
     b() {
       print("b1");
     }
-  .packages: example:pkg/example
+  .dart_tool/package_config.json: |
+    {
+      "configVersion": 2,
+      "packages": [
+        {
+          "name": "example",
+          "rootUri": "../pkg/example"
+        }
+      ]
+    }
diff --git a/pkg/front_end/testcases/incremental/invalidate_package_part_as_package.yaml b/pkg/front_end/testcases/incremental/invalidate_package_part_as_package.yaml
index 68e2d37ffbb..6519fd4b4dd 100644
--- a/pkg/front_end/testcases/incremental/invalidate_package_part_as_package.yaml
+++ b/pkg/front_end/testcases/incremental/invalidate_package_part_as_package.yaml
@@ -22,4 +22,13 @@ sources:
     b() {
       print("b1");
     }
-  .packages: example:pkg/example
+  .dart_tool/package_config.json: |
+    {
+      "configVersion": 2,
+      "packages": [
+        {
+          "name": "example",
+          "rootUri": "../pkg/example"
+        }
+      ]
+    }
diff --git a/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_file.yaml b/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_file.yaml
index ee3691451a7..17c374abf6e 100644
--- a/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_file.yaml
+++ b/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_file.yaml
@@ -23,4 +23,13 @@ sources:
     b() {
       print("b1");
     }
-  .packages: example:pkg/example
+  .dart_tool/package_config.json: |
+    {
+      "configVersion": 2,
+      "packages": [
+        {
+          "name": "example",
+          "rootUri": "../pkg/example"
+        }
+      ]
+    }
diff --git a/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_package.yaml b/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_package.yaml
index 5af601b77b8..d79692eaf03 100644
--- a/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_package.yaml
+++ b/pkg/front_end/testcases/incremental/invalidate_package_part_from_package_url_as_package.yaml
@@ -23,4 +23,13 @@ sources:
     b() {
       print("b1");
     }
-  .packages: example:pkg/example
+  .dart_tool/package_config.json: |
+    {
+      "configVersion": 2,
+      "packages": [
+        {
+          "name": "example",
+          "rootUri": "../pkg/example"
+        }
+      ]
+    }
diff --git a/pkg/front_end/testcases/incremental/no_outline_change_27.yaml b/pkg/front_end/testcases/incremental/no_outline_change_27.yaml
index 8f4c25d06ba..380c552b914 100644
--- a/pkg/front_end/testcases/incremental/no_outline_change_27.yaml
+++ b/pkg/front_end/testcases/incremental/no_outline_change_27.yaml
@@ -22,8 +22,16 @@ worlds:
         libMethod() {
           print("Hello from lib!");
         }
-      .packages: |
-        foo:.
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": ".."
+            }
+          ]
+        }
     expectedLibraryCount: 2
   - entry: main.dart
     warnings: false
diff --git a/pkg/front_end/testcases/incremental/part_as_package_entry.yaml b/pkg/front_end/testcases/incremental/part_as_package_entry.yaml
index 9aed0c25229..9810036ccd1 100644
--- a/pkg/front_end/testcases/incremental/part_as_package_entry.yaml
+++ b/pkg/front_end/testcases/incremental/part_as_package_entry.yaml
@@ -7,8 +7,16 @@ worlds:
   - entry: package:foo/main.dart
     checkEntries: false
     sources:
-      .packages: |
-        foo:.
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": ".."
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         part of 'lib.dart';
diff --git a/pkg/front_end/testcases/incremental/part_as_package_entry_2.yaml b/pkg/front_end/testcases/incremental/part_as_package_entry_2.yaml
index 27bb66ca4fa..2ac3ec6f285 100644
--- a/pkg/front_end/testcases/incremental/part_as_package_entry_2.yaml
+++ b/pkg/front_end/testcases/incremental/part_as_package_entry_2.yaml
@@ -8,8 +8,16 @@ worlds:
     warnings: false
     checkEntries: false
     sources:
-      .packages: |
-        foo:.
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": ".."
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         part of 'lib.dart';
diff --git a/pkg/front_end/testcases/incremental/part_not_part_of_as_package.yaml b/pkg/front_end/testcases/incremental/part_not_part_of_as_package.yaml
index 91106066380..a3c62ed58a5 100644
--- a/pkg/front_end/testcases/incremental/part_not_part_of_as_package.yaml
+++ b/pkg/front_end/testcases/incremental/part_not_part_of_as_package.yaml
@@ -7,8 +7,16 @@ worlds:
   - entry: package:foo/main.dart
     errors: true
     sources:
-      .packages: |
-        foo:.
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "foo",
+              "rootUri": ".."
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         part 'lib.dart';
diff --git a/pkg/front_end/testcases/incremental/reissue_errors_3.yaml b/pkg/front_end/testcases/incremental/reissue_errors_3.yaml
index b93e15d902e..fc0ac604a27 100644
--- a/pkg/front_end/testcases/incremental/reissue_errors_3.yaml
+++ b/pkg/front_end/testcases/incremental/reissue_errors_3.yaml
@@ -10,7 +10,16 @@ type: newworld
 worlds:
   - entry: main.dart
     sources:
-      .packages: mypackage:mypackage
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "mypackage",
+              "rootUri": "../mypackage"
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         import "package:mypackage/a.dart";
@@ -34,7 +43,16 @@ worlds:
     expectedLibraryCount: 3
   - entry: main.dart
     sources:
-      .packages: mypackage:mypackage
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "mypackage",
+              "rootUri": "../mypackage"
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         import "package:mypackage/a.dart";
diff --git a/pkg/front_end/testcases/incremental/reissue_errors_4.yaml b/pkg/front_end/testcases/incremental/reissue_errors_4.yaml
index 47f97882bc7..b53906c9e2c 100644
--- a/pkg/front_end/testcases/incremental/reissue_errors_4.yaml
+++ b/pkg/front_end/testcases/incremental/reissue_errors_4.yaml
@@ -10,7 +10,16 @@ type: newworld
 worlds:
   - entry: main.dart
     sources:
-      .packages: mypackage:mypackage
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "mypackage",
+              "rootUri": "../mypackage"
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         import "package:mypackage/a.dart";
@@ -38,7 +47,16 @@ worlds:
     expectedLibraryCount: 3
   - entry: main.dart
     sources:
-      .packages: mypackage:mypackage
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "mypackage",
+              "rootUri": "../mypackage"
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         import "package:mypackage/a.dart";
diff --git a/pkg/front_end/testcases/incremental/reissue_errors_5.yaml b/pkg/front_end/testcases/incremental/reissue_errors_5.yaml
index 36e836191dc..861b204e3cd 100644
--- a/pkg/front_end/testcases/incremental/reissue_errors_5.yaml
+++ b/pkg/front_end/testcases/incremental/reissue_errors_5.yaml
@@ -10,7 +10,16 @@ type: newworld
 worlds:
   - entry: main.dart
     sources:
-      .packages: mypackage:mypackage
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "mypackage",
+              "rootUri": "../mypackage"
+            }
+          ]
+        }
       main.dart: |
         // @dart=2.9
         import "package:mypackage/a.dart" as a;
diff --git a/pkg/front_end/testcases/incremental/updated_package_1.yaml b/pkg/front_end/testcases/incremental/updated_package_1.yaml
index 6950f986c30..67c83b06b1f 100644
--- a/pkg/front_end/testcases/incremental/updated_package_1.yaml
+++ b/pkg/front_end/testcases/incremental/updated_package_1.yaml
@@ -3,7 +3,7 @@
 # BSD-style license that can be found in the LICENSE.md file.
 
 # Compile an application with a package and use it.
-# Then update the package from .packages.
+# Then update the package from .dart_tool/package_config.json.
 # The old package is still included in the dill file we initialize from,
 # but shouldn't cause trouble, nor be included in the output.
 
@@ -32,7 +32,16 @@ worlds:
         b() {
           a();
         }
-      .packages: example:package_0.1.0
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.0"
+            }
+          ]
+        }
     expectedLibraryCount: 3
   - entry: main.dart
     # TODO(jensj): For now we don't initialize from dill when a package was
@@ -67,5 +76,14 @@ worlds:
         b() {
           print("hello from v0.1.1");
         }
-      .packages: example:package_0.1.1
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.1"
+            }
+          ]
+        }
     expectedLibraryCount: 2
diff --git a/pkg/front_end/testcases/incremental/updated_package_2.yaml b/pkg/front_end/testcases/incremental/updated_package_2.yaml
index 34110bab106..6e7ae115cbd 100644
--- a/pkg/front_end/testcases/incremental/updated_package_2.yaml
+++ b/pkg/front_end/testcases/incremental/updated_package_2.yaml
@@ -3,7 +3,7 @@
 # BSD-style license that can be found in the LICENSE.md file.
 
 # Compile an application with a package and use it.
-# Then update the package from .packages.
+# Then update the package from .dart_tool/package_config.json.
 # The old package is still included in the dill file we initialize from,
 # but shouldn't cause trouble, nor be included in the output.
 
@@ -32,7 +32,16 @@ worlds:
         b() {
           a();
         }
-      .packages: example:package_0.1.0
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.0"
+            }
+          ]
+        }
     expectedLibraryCount: 3
   - entry: main.dart
     invalidate:
@@ -66,5 +75,14 @@ worlds:
         b() {
           print("hello from v0.1.1");
         }
-      .packages: example:package_0.1.1
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.1"
+            }
+          ]
+        }
     expectedLibraryCount: 2
diff --git a/pkg/front_end/testcases/incremental/updated_package_3.yaml b/pkg/front_end/testcases/incremental/updated_package_3.yaml
index faf431eefec..e3b97b717ae 100644
--- a/pkg/front_end/testcases/incremental/updated_package_3.yaml
+++ b/pkg/front_end/testcases/incremental/updated_package_3.yaml
@@ -3,8 +3,8 @@
 # BSD-style license that can be found in the LICENSE.md file.
 
 # Compile an application with a package and use it.
-# Then update the package from .packages but change nothing else and call
-# computeDelta on the same compiler.
+# Then update the package from .dart_tool/package_config.json but change nothing
+# else and call computeDelta on the same compiler.
 # The output should not include the old package.
 
 type: newworld
@@ -32,7 +32,16 @@ worlds:
         b() {
           a();
         }
-      .packages: example:package_0.1.0
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.0"
+            }
+          ]
+        }
     expectedLibraryCount: 3
   - entry: main.dart
     worldType: updated
@@ -46,5 +55,14 @@ worlds:
         b() {
           print("hello from v0.1.1");
         }
-      .packages: example:package_0.1.1
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.1"
+            }
+          ]
+        }
     expectedLibraryCount: 2
diff --git a/pkg/front_end/testcases/incremental/updated_package_4.yaml b/pkg/front_end/testcases/incremental/updated_package_4.yaml
index 4d2fd14f684..e5d21b35d9c 100644
--- a/pkg/front_end/testcases/incremental/updated_package_4.yaml
+++ b/pkg/front_end/testcases/incremental/updated_package_4.yaml
@@ -2,9 +2,9 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE.md file.
 
-# Compile an application with a .packages.
-# Then delete the content of .packages, still using the package in code.
-# This should be an error.
+# Compile an application with a .dart_tool/package_config.json.
+# Then delete the content of .dart_tool/package_config.json, still using the
+# package in code. This should be an error.
 
 type: newworld
 worlds:
@@ -22,15 +22,28 @@ worlds:
         b() {
           print("hello from package");
         }
-      .packages: example:package
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package"
+            }
+          ]
+        }
     expectedLibraryCount: 2
   - entry: main.dart
     worldType: updated
     expectInitializeFromDill: false
     checkInvalidatedFiles: false
     invalidate:
-      - .packages
+      - .dart_tool/package_config.json
     sources:
-      .packages:
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": []
+        }
     errors: true
     expectedLibraryCount: 1
diff --git a/pkg/front_end/testcases/incremental/updated_package_uri.yaml b/pkg/front_end/testcases/incremental/updated_package_uri.yaml
index f8e7ad9078d..53d17254dff 100644
--- a/pkg/front_end/testcases/incremental/updated_package_uri.yaml
+++ b/pkg/front_end/testcases/incremental/updated_package_uri.yaml
@@ -2,9 +2,10 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE.md file.
 
-# Compile an application with a .packages file.
-# Then change which file should be the .packages file to use, and everything
-# should now be relative to the new .packages file.
+# Compile an application with a .dart_tool/package_config.json file.
+# Then change which file should be the package_config.json file to
+# use, and everything should now be relative to the new
+# package_config.json file.
 
 type: newworld
 worlds:
@@ -31,7 +32,16 @@ worlds:
         b() {
           a();
         }
-      .packages: example:package_0.1.0
+      .dart_tool/package_config.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.0"
+            }
+          ]
+        }
     expectedLibraryCount: 3
   - entry: main.dart
     worldType: updated
@@ -42,6 +52,15 @@ worlds:
         b() {
           print("hello from v0.1.1");
         }
-      .packages2: example:package_0.1.1
-    dotPackagesFile: .packages2
+      .dart_tool/package_config2.json: |
+        {
+          "configVersion": 2,
+          "packages": [
+            {
+              "name": "example",
+              "rootUri": "../package_0.1.1"
+            }
+          ]
+        }
+    packageConfigFile: .dart_tool/package_config2.json
     expectedLibraryCount: 2
diff --git a/pkg/front_end/tool/fasta_perf.dart b/pkg/front_end/tool/fasta_perf.dart
index 6c5a003f56b..ba61cb1ec69 100644
--- a/pkg/front_end/tool/fasta_perf.dart
+++ b/pkg/front_end/tool/fasta_perf.dart
@@ -90,7 +90,7 @@ Future setup(Uri entryUri) async {
     // allowlisting of error messages in the error handler.
     ..onDiagnostic = onDiagnosticMessageHandler()
     ..compileSdk = true
-    ..packagesFileUri = Uri.base.resolve('.packages')
+    ..packagesFileUri = Uri.base.resolve('.dart_tool/package_config.json')
     ..target = createTarget(isFlutter: false);
   uriResolver = await new ProcessedOptions(options: options).getUriTranslator();
 }
@@ -231,7 +231,7 @@ Future<CompilerResult> generateKernel(Uri entryUri,
     ..sdkRoot = sdkRoot
     ..onDiagnostic = onDiagnosticMessageHandler()
     ..target = createTarget(isFlutter: false)
-    ..packagesFileUri = Uri.base.resolve('.packages')
+    ..packagesFileUri = Uri.base.resolve('.dart_tool/package_config.json')
     ..compileSdk = compileSdk
     ..environmentDefines = const {};
   if (!compileSdk) {
diff --git a/pkg/front_end/tool/incremental_perf.dart b/pkg/front_end/tool/incremental_perf.dart
index 0a1186dbc22..dc1c3bbbad7 100644
--- a/pkg/front_end/tool/incremental_perf.dart
+++ b/pkg/front_end/tool/incremental_perf.dart
@@ -215,9 +215,9 @@ class OverlayFileSystem implements FileSystem {
     if (uri.isScheme('org-dartlang-overlay')) {
       return new OverlayFileSystemEntity(uri, this);
     } else if (uri.isScheme('file')) {
-      // The IKG compiler reads ".packages" which might contain absolute file
-      // URIs (which it will then try to use on the FS).  We therefore replace
-      // them with overlay-fs URIs as usual.
+      // The IKG compiler reads ".dart_tool/package_config.json" which might
+      // contain absolute file URIs (which it will then try to use on the FS).
+      // We therefore replace them with overlay-fs URIs as usual.
       return new OverlayFileSystemEntity(_resolveOverlayUri('$uri'), this);
     } else {
       throw "Unsupported scheme: ${uri.scheme}."