diff --git a/pkg/front_end/lib/src/fasta/type_inference/closure_context.dart b/pkg/front_end/lib/src/fasta/type_inference/closure_context.dart
index 161109e8051..f8373f8fa8f 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/closure_context.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/closure_context.dart
@@ -427,7 +427,7 @@ class _SyncClosureContext implements ClosureContext {
       returnType = _declaredReturnType;
     }
     if (inferrer.library.isNonNullableByDefault &&
-        (containsInvalidType(returnType) ||
+        (!containsInvalidType(returnType) &&
             returnType.isPotentiallyNonNullable) &&
         inferrer.flowAnalysis.isReachable) {
       Statement resultStatement =
@@ -830,7 +830,7 @@ class _AsyncClosureContext implements ClosureContext {
     }
     returnType = inferrer.typeSchemaEnvironment.flatten(returnType);
     if (inferrer.library.isNonNullableByDefault &&
-        (containsInvalidType(returnType) ||
+        (!containsInvalidType(returnType) &&
             returnType.isPotentiallyNonNullable) &&
         inferrer.flowAnalysis.isReachable) {
       Statement resultStatement =
diff --git a/pkg/front_end/messages.status b/pkg/front_end/messages.status
index 17b04d67226..624830e8aaa 100644
--- a/pkg/front_end/messages.status
+++ b/pkg/front_end/messages.status
@@ -744,13 +744,8 @@ SuperclassMethodArgumentMismatch/analyzerCode: Fail
 SuperclassMethodArgumentMismatch/example: Fail
 SupertypeIsFunction/analyzerCode: Fail
 SupertypeIsFunction/example: Fail
-SupertypeIsIllegal/example: Fail
-SupertypeIsIllegalAliased/example: Fail
-SupertypeIsNullableAliased/example: Fail
-SupertypeIsTypeVariable/example: Fail
 SwitchCaseFallThrough/example: Fail
 SwitchExpressionNotSubtype/analyzerCode: Fail
-SwitchExpressionNotSubtype/example: Fail
 SwitchExpressionUserDefinedEquals/analyzerCode: Fail
 SwitchExpressionUserDefinedEquals/example: Fail
 SyntheticToken/example: Fail # Can't be tested, used to recover from other errors.
@@ -786,7 +781,6 @@ UndefinedExtensionOperator/analyzerCode: Fail
 UndefinedExtensionOperator/example: Fail
 UndefinedExtensionSetter/analyzerCode: Fail
 UndefinedExtensionSetter/example: Fail
-UnexpectedModifierInNonNnbd/part_wrapped_script: Fail # Test requires @dart= annotation
 UnexpectedToken/part_wrapped_script1: Fail
 UnexpectedToken/script1: Fail
 UnmatchedToken/part_wrapped_script1: Fail
diff --git a/pkg/front_end/messages.yaml b/pkg/front_end/messages.yaml
index 05f2d165fe4..b1ae496a901 100644
--- a/pkg/front_end/messages.yaml
+++ b/pkg/front_end/messages.yaml
@@ -14,6 +14,9 @@
 #    should be the path to an external test. The external test will not be run,
 #    but the existence of the file will be verified.
 #
+#    Multiple scripts can start with a `// @dart=` annotation to enforce the
+#    language version used for the example code.
+#
 # Note that it can be hard or impossible to write an example that only gives the
 # specific error. To allow for this, one can specify
 # "exampleAllowMoreCodes: true" which filters out every message with a
@@ -588,12 +591,20 @@ AbstractClassMember:
   tip: "Try removing the 'abstract' keyword. You can add the 'abstract' keyword before the class declaration."
   analyzerCode: ParserErrorCode.ABSTRACT_CLASS_MEMBER
   script:
-    - "abstract class C {abstract C.c();}"
-    - "abstract class C {abstract m();}"
-    - "abstract class C {abstract get m;}"
-    - "abstract class C {abstract set m(int x);}"
-    - "abstract class C {abstract var f;}"
-    - "abstract class C {abstract static var f;}"
+    - |
+      abstract class C {abstract C.c();}
+    - |
+      abstract class C {abstract m();}
+    - |
+      abstract class C {abstract get m;}
+    - |
+      abstract class C {abstract set m(int x);}
+    - |
+      // @dart=2.9
+      abstract class C {abstract var f;}
+    - |
+      // @dart=2.9
+      abstract class C {abstract static var f;}
 
 AbstractExternalField:
   index: 110
@@ -720,8 +731,11 @@ ConstFactory:
   template: "Only redirecting factory constructors can be declared to be 'const'."
   tip: "Try removing the 'const' keyword, or replacing the body with '=' followed by a valid target."
   analyzerCode: ParserErrorCode.CONST_FACTORY
-  script:
-    - "class C { const factory C() {} }"
+  script: |
+    class C {
+      const factory C() => const C.internal();
+      const C.internal();
+    }
 
 ConstFactoryRedirectionToNonConst:
   template: "Constant factory constructor can't delegate to a non-constant constructor."
@@ -762,8 +776,11 @@ TypeBeforeFactory:
   template: "Factory constructors cannot have a return type."
   tip: "Try removing the type appearing before 'factory'."
   analyzerCode: ParserErrorCode.TYPE_BEFORE_FACTORY
-  script:
-    - "class C { T factory C() { return null; } }"
+  script: |
+    class C {
+      T factory C() { return new C.constructor(); }
+      C.constructor();
+    }
 
 ConstConstructorWithBody:
   template: "A const constructor can't have a body."
@@ -828,7 +845,9 @@ ExternalField:
   tip: "Try removing the keyword 'external', or replacing the field by an external getter and/or setter."
   analyzerCode: ParserErrorCode.EXTERNAL_FIELD
   script:
-    - "class C { external var f; }"
+    - |
+      // @dart=2.9
+      class C { external var f; }
 
 
 ExternalFieldInitializer:
@@ -885,10 +904,10 @@ RedirectingConstructorWithAnotherInitializer:
   # also ASSERT_IN_REDIRECTING_CONSTRUCTOR
   analyzerCode: FIELD_INITIALIZER_REDIRECTING_CONSTRUCTOR
   script:
-    - "class C { int x; C(); C.bad() : x = 5, this(); }"
-    - "class C { int x; C(); C.bad() : this(), x = 5; }"
-    - "class C { int x; C(); C.bad() : assert(true), this(); }"
-    - "class C { int x; C(); C.bad() : this(), assert(true); }"
+    - "class C { int? x; C(); C.bad() : x = 5, this(); }"
+    - "class C { int? x; C(); C.bad() : this(), x = 5; }"
+    - "class C { int? x; C(); C.bad() : assert(true), this(); }"
+    - "class C { int? x; C(); C.bad() : this(), assert(true); }"
 
 SuperInitializerNotLast:
   template: "Can't have initializers after 'super'."
@@ -1575,10 +1594,20 @@ ConstructorWithTypeParameters:
   analyzerCode: ParserErrorCode.TYPE_PARAMETER_ON_CONSTRUCTOR
   tip: "Try removing the type parameters."
   script:
-    - "class C { C<T>() {} }"
-    - "class C { C.foo<T>() {} }"
-    - "class C { factory C<T>() => null; }"
-    - "class C { factory C.foo<T>() => null; }"
+    - >-
+      class C { C<T>() {} }
+    - >-
+      class C { C.foo<T>() {} }
+    - >-
+      class C {
+        factory C<T>() => new C.internal();
+        C.internal();
+      }
+    - >-
+      class C {
+        factory C.foo<T>() => new C.internal();
+        C.internal();
+      }
 
 ConstructorWithTypeArguments:
   template: "A constructor invocation can't have type arguments on the constructor name."
@@ -1593,9 +1622,18 @@ ConstructorWithWrongName:
   analyzerCode: ParserErrorCode.INVALID_CONSTRUCTOR_NAME
   index: 102
   script:
-    - "class A { B.foo() {} }"
-    - "class A { factory B() => null; }"
-    - "class A { factory B.foo() => null; }"
+    - >-
+      class A { B.foo() {} }
+    - >-
+      class A {
+        factory B() => new A.internal();
+        A.internal();
+      }
+    - >-
+      class A {
+        factory B.foo() => new A.internal();
+        A.internal();
+      }
 
 ConstructorWithWrongNameContext:
   template: "The name of the enclosing class is '#name'."
@@ -2379,6 +2417,7 @@ InvalidGetterSetterTypeBothInheritedGetter:
 InvalidGetterSetterTypeLegacy:
   template: "The type '#type' of the getter '#name' is not assignable to the type '#type2' of the setter '#name2'."
   script: |
+    // @dart=2.9
     abstract class A {
       String get property;
       void set property(int i);
@@ -2387,6 +2426,7 @@ InvalidGetterSetterTypeLegacy:
 InvalidGetterSetterTypeGetterInheritedLegacy:
   template: "The type '#type' of the inherited getter '#name' is not assignable to the type '#type2' of the setter '#name2'."
   script: |
+    // @dart=2.9
     abstract class A {
       String get property;
     }
@@ -2397,6 +2437,7 @@ InvalidGetterSetterTypeGetterInheritedLegacy:
 InvalidGetterSetterTypeFieldInheritedLegacy:
   template: "The type '#type' of the inherited field '#name' is not assignable to the type '#type2' of the setter '#name2'."
   script: |
+    // @dart=2.9
     abstract class A {
       final String property;
       A(this.property);
@@ -2408,6 +2449,7 @@ InvalidGetterSetterTypeFieldInheritedLegacy:
 InvalidGetterSetterTypeSetterInheritedGetterLegacy:
   template: "The type '#type' of the getter '#name' is not assignable to the type '#type2' of the inherited setter '#name2'."
   script: |
+    // @dart=2.9
     abstract class A {
       void set property(int i);
     }
@@ -2418,6 +2460,7 @@ InvalidGetterSetterTypeSetterInheritedGetterLegacy:
 InvalidGetterSetterTypeSetterInheritedFieldLegacy:
   template: "The type '#type' of the field '#name' is not assignable to the type '#type2' of the inherited setter '#name2'."
   script: |
+    // @dart=2.9
     abstract class A {
       void set property(int i);
     }
@@ -2429,6 +2472,7 @@ InvalidGetterSetterTypeSetterInheritedFieldLegacy:
 InvalidGetterSetterTypeBothInheritedFieldLegacy:
   template: "The type '#type' of the inherited field '#name' is not assignable to the type '#type2' of the inherited setter '#name2'."
   script: |
+    // @dart=2.9
     abstract class A {
       final String property;
       A(this.property);
@@ -2441,6 +2485,7 @@ InvalidGetterSetterTypeBothInheritedFieldLegacy:
 InvalidGetterSetterTypeBothInheritedGetterLegacy:
   template: "The type '#type' of the inherited getter '#name' is not assignable to the type '#type2' of the inherited setter '#name2'."
   script: |
+    // @dart=2.9
     abstract class A {
       String get property;
     }
@@ -2535,8 +2580,8 @@ ExternalConstructorWithInitializer:
   template: "An external constructor can't have any initializers."
   analyzerCode: ParserErrorCode.EXTERNAL_CONSTRUCTOR_WITH_INITIALIZER
   script:
-    - "class C { int x; external C() : x = 1; }"
-    - "class C { int x; external C.foo() : x = 1; }"
+    - "class C { int? x; external C() : x = 1; }"
+    - "class C { int? x; external C.foo() : x = 1; }"
 
 ExternalTypedef:
   index: 76
@@ -2578,7 +2623,9 @@ TypedefTypeVariableNotConstructorCause:
 TypedefNullableType:
   template: "Can't create typedef from nullable type."
   configuration: nnbd-strong
-  script: typedef F = void Function()?;
+  script: |
+    // @dart=2.12
+    typedef F = void Function()?;
 
 TypedefUnaliasedTypeCause:
   template: "This is the type denoted by the type alias."
@@ -2898,8 +2945,8 @@ MemberWithSameNameAsClass:
     - "class C { get C => 42; }"
     - "class C { set C(x) {} }"
     - "class C { set C(x) => 42; }"
-    - "class C { int C; }"
-    - "class C { int A, B, C, D, E; }"
+    - "class C { int? C; }"
+    - "class C { int? A, B, C, D, E; }"
 
 EnumConstantSameNameAsEnclosing:
   template: "Name of enum constant '#name' can't be the same as the enum's own name."
@@ -2951,18 +2998,30 @@ OperatorMinusParameterMismatch:
 SupertypeIsIllegal:
   template: "The type '#name' can't be used as supertype."
   analyzerCode: EXTENDS_NON_CLASS
+  script: |
+    class C extends dynamic {}
 
 SupertypeIsIllegalAliased:
   template: "The type '#name' which is an alias of '#type' can't be used as supertype."
   analyzerCode: EXTENDS_NON_CLASS
+  script: |
+    typedef F = void Function();
+    class C extends F {}
 
 SupertypeIsNullableAliased:
   template: "The type '#name' which is an alias of '#type' can't be used as supertype because it is nullable."
   analyzerCode: EXTENDS_NON_CLASS
+  experiments: nonfunction-type-aliases
+  script: |
+    class A {}
+    typedef B = A?;
+    class C extends B {}
 
 SupertypeIsTypeVariable:
   template: "The type variable '#name' can't be used as supertype."
   analyzerCode: EXTENDS_NON_CLASS
+  script: |
+    class C<T> extends T {}
 
 PartOfLibraryNameMismatch:
   template: "Using '#uri' as part of '#name' but its 'part of' declaration says '#name2'."
@@ -3033,7 +3092,8 @@ SwitchExpressionNotAssignable:
   template: "Type '#type' of the switch expression isn't assignable to the type '#type2' of this case expression."
   analyzerCode: SWITCH_EXPRESSION_NOT_ASSIGNABLE
   script:
-    - >-
+    - |
+      // @dart=2.9
       void f() {
         switch (42) {
           case "foo": break;
@@ -3046,6 +3106,13 @@ SwitchExpressionNotAssignableCause:
 
 SwitchExpressionNotSubtype:
   template: "Type '#type' of the case expression is not a subtype of type '#type2' of this switch expression."
+  script:
+    - |
+      void f() {
+        switch (42) {
+          case "foo": break;
+        }
+      }
 
 SwitchHasCaseAfterDefault:
   index: 16
@@ -3107,14 +3174,14 @@ TypeVariableInStaticContext:
     - |
       class C<T> {
         static staticMethod() {
-          List<T> t = null;
+          List<T>? t = null;
           return t;
         }
       }
     - |
       class C<T> {
         static staticMethod() {
-          T t = null;
+          T? t = null;
           return t;
         }
       }
@@ -3132,13 +3199,13 @@ TypeVariableInConstantContext:
     - |
       class C<T> {
         instanceMethod() {
-          return const <T>[null];
+          return const <T>[];
         }
       }
     - |
       class C<T> {
         instanceMethod() {
-          return const <List<T>>[null];
+          return const <List<T>>[];
         }
       }
 
@@ -3509,8 +3576,7 @@ UndefinedGetter:
   analyzerCode: UNDEFINED_GETTER
   script: >
     class C {}
-    main() {
-      C c;
+    test(C c) {
       print(c.foo);
     }
 
@@ -3520,8 +3586,7 @@ UndefinedSetter:
   analyzerCode: UNDEFINED_SETTER
   script: >
     class C {}
-    main() {
-      C c;
+    test(C c) {
       c.foo = 0;
     }
 
@@ -3531,8 +3596,7 @@ UndefinedMethod:
   analyzerCode: UNDEFINED_METHOD
   script: >
     class C {}
-    main() {
-      C c;
+    test(C c) {
       c.foo();
     }
 
@@ -3542,8 +3606,7 @@ UndefinedOperator:
   analyzerCode: UNDEFINED_METHOD
   script: >
     class C {}
-    main() {
-      C c;
+    test(C c) {
       c + 0;
     }
 
@@ -3947,7 +4010,9 @@ ReturnWithoutExpression:
   template: "Must explicitly return a value from a non-void function."
   severity: WARNING
   analyzerCode: RETURN_WITHOUT_VALUE
-  declaration: "int foo() { return; }"
+  script: |
+    // @dart=2.9
+    int foo() { return; }
 
 ReturnWithoutExpressionSync:
   template: "A value must be explicitly returned from a non-void function."
@@ -4025,6 +4090,7 @@ InvokeNonFunction:
   template: "'#name' isn't a function or method and can't be invoked."
   analyzerCode: INVOCATION_OF_NON_FUNCTION
   script: |
+    // @dart=2.9
     class Foo {
       int f;
     }
@@ -4100,8 +4166,8 @@ IncompatibleRedirecteeFunctionType:
       }
     - >-
       class A {
-        factory A.f({int x}) = A.g;
-        A.g({int y}) { }
+        factory A.f({int? x}) = A.g;
+        A.g({int? y}) { }
       }
     - >-
       class A {
@@ -4159,7 +4225,7 @@ IncorrectTypeArgumentInReturnType:
   analyzerCode: TYPE_ARGUMENT_NOT_MATCHING_BOUNDS
   script: >
     class A<T extends num> {}
-    A<String> foo() => null;
+    A<String> foo() => throw '';
 
 IncorrectTypeArgumentInferred:
   template: "Inferred type argument '#type' doesn't conform to the bound '#type2' of the type variable '#name' on '#name2'."
@@ -4217,14 +4283,16 @@ GenericFunctionTypeUsedAsActualTypeArgument:
   analyzerCode: GENERIC_FUNCTION_CANNOT_BE_TYPE_ARGUMENT
   script:
     - >-
-      typedef F = List<T> Function<T>(T);
+      typedef F = Class<T> Function<T>(T);
+      class Class<T> {}
       main() {
-        List<F> list1;
+        Class<F> class1;
       }
     - >-
-      typedef F = List<T> Function<T>(T);
+      typedef F = Class<T> Function<T>(T);
+      class Class<T> {}
       main() {
-        new List<F>();
+        Class<F>();
       }
 
 GenericFunctionTypeInferredAsActualTypeArgument:
@@ -4400,7 +4468,7 @@ SpreadTypeMismatch:
       }
     - |
       main() {
-        int Function() a = null;
+        int Function() a = () => 42;
         var b = [...a];
       }
 
@@ -4443,7 +4511,7 @@ SpreadMapEntryTypeMismatch:
       }
     - |
       main() {
-        int Function() a = null;
+        int Function() a = () => 42;
         var b = <dynamic, dynamic>{...a};
       }
 
diff --git a/pkg/front_end/test/fasta/messages_suite.dart b/pkg/front_end/test/fasta/messages_suite.dart
index 9795ed40d5d..75300429497 100644
--- a/pkg/front_end/test/fasta/messages_suite.dart
+++ b/pkg/front_end/test/fasta/messages_suite.dart
@@ -28,10 +28,14 @@ import "package:vm/target/vm.dart" show VmTarget;
 import "package:yaml/yaml.dart" show YamlList, YamlMap, YamlNode, loadYamlNode;
 
 import 'package:front_end/src/api_prototype/compiler_options.dart'
-    show CompilerOptions, InvocationMode;
+    show
+        CompilerOptions,
+        InvocationMode,
+        parseExperimentalArguments,
+        parseExperimentalFlags;
 
 import 'package:front_end/src/api_prototype/experimental_flags.dart'
-    show ExperimentalFlag;
+    show ExperimentalFlag, defaultExperimentalFlags;
 
 import 'package:front_end/src/api_prototype/memory_file_system.dart'
     show MemoryFileSystem;
@@ -78,10 +82,7 @@ class Configuration {
 
   CompilerOptions apply(CompilerOptions options) {
     if (nnbdMode != null) {
-      options.explicitExperimentalFlags[ExperimentalFlag.nonNullable] = true;
       options.nnbdMode = nnbdMode;
-    } else {
-      options.explicitExperimentalFlags[ExperimentalFlag.nonNullable] = false;
     }
     options.invocationModes = invocationModes;
     return options;
@@ -169,6 +170,7 @@ class MessageTestSuite extends ChainContext {
           "'spell_checking_list_messages.txt' or "
           "'spell_checking_list_common.txt'.";
       Configuration configuration;
+      Map<ExperimentalFlag, bool> experimentalFlags;
 
       Source source;
       List<String> formatSpellingMistakes(spell.SpellingResult spellResult,
@@ -372,6 +374,16 @@ class MessageTestSuite extends ChainContext {
             }
             break;
 
+          case "experiments":
+            if (value is String) {
+              experimentalFlags = parseExperimentalFlags(
+                  parseExperimentalArguments(value.split(',')),
+                  onError: (message) => throw new ArgumentError(message));
+            } else {
+              throw new ArgumentError("Unknown experiments value: $value.");
+            }
+            break;
+
           default:
             unknownKeys.add(key);
         }
@@ -386,6 +398,8 @@ class MessageTestSuite extends ChainContext {
       for (Example example in examples) {
         example.configuration =
             configuration ?? Configuration.defaultConfiguration;
+        example.experimentalFlags =
+            experimentalFlags ?? defaultExperimentalFlags;
       }
 
       MessageTestDescription createDescription(
@@ -527,15 +541,13 @@ abstract class Example {
 
   Configuration configuration;
 
+  Map<ExperimentalFlag, bool> experimentalFlags;
+
   Example(this.name, this.expectedCode);
 
   YamlNode get node;
 
-  Uint8List get bytes;
-
-  Map<String, Uint8List> get scripts {
-    return {mainFilename: bytes};
-  }
+  Map<String, Script> get scripts;
 
   String get mainFilename => "main.dart";
 }
@@ -544,12 +556,16 @@ class BytesExample extends Example {
   @override
   final YamlList node;
 
-  @override
   final Uint8List bytes;
 
   BytesExample(String name, String code, this.node)
       : bytes = new Uint8List.fromList(node.cast<int>()),
         super(name, code);
+
+  @override
+  Map<String, Script> get scripts {
+    return {mainFilename: new Script(bytes, '', null)};
+  }
 }
 
 class DeclarationExample extends Example {
@@ -563,13 +579,15 @@ class DeclarationExample extends Example {
         super(name, code);
 
   @override
-  Uint8List get bytes {
-    return new Uint8List.fromList(utf8.encode("""
+  Map<String, Script> get scripts {
+    return {
+      mainFilename: new Script.fromSource("""
 $declaration
 
 main() {
 }
-"""));
+""")
+    };
   }
 }
 
@@ -584,12 +602,14 @@ class StatementExample extends Example {
         super(name, code);
 
   @override
-  Uint8List get bytes {
-    return new Uint8List.fromList(utf8.encode("""
+  Map<String, Script> get scripts {
+    return {
+      mainFilename: new Script.fromSource("""
 main() {
   $statement
 }
-"""));
+""")
+    };
   }
 }
 
@@ -603,13 +623,14 @@ class ExpressionExample extends Example {
       : expression = node.value,
         super(name, code);
 
-  @override
-  Uint8List get bytes {
-    return new Uint8List.fromList(utf8.encode("""
+  Map<String, Script> get scripts {
+    return {
+      mainFilename: new Script.fromSource("""
 main() {
   $expression;
 }
-"""));
+""")
+    };
   }
 }
 
@@ -630,20 +651,17 @@ class ScriptExample extends Example {
   }
 
   @override
-  Uint8List get bytes => throw "Unsupported: ScriptExample.bytes";
-
-  @override
-  Map<String, Uint8List> get scripts {
+  Map<String, Script> get scripts {
     Object script = this.script;
     if (script is Map) {
-      var scriptFiles = <String, Uint8List>{};
+      Map<String, Script> scriptFiles = <String, Script>{};
       script.forEach((fileName, value) {
-        scriptFiles[fileName] = new Uint8List.fromList(utf8.encode(value));
+        scriptFiles[fileName] = new Script.fromSource(value);
         print("$fileName => $value\n\n======\n\n");
       });
       return scriptFiles;
     } else {
-      return {mainFilename: new Uint8List.fromList(utf8.encode(script))};
+      return {mainFilename: new Script.fromSource(script)};
     }
   }
 }
@@ -656,19 +674,17 @@ class PartWrapExample extends Example {
   PartWrapExample(String name, String code, this.allowMoreCodes, this.example)
       : super(name, code) {
     configuration = example.configuration;
+    experimentalFlags = example.experimentalFlags;
   }
 
-  @override
-  Uint8List get bytes => throw "Unsupported: PartWrapExample.bytes";
-
   @override
   String get mainFilename => "main_wrapped.dart";
 
   @override
-  Map<String, Uint8List> get scripts {
-    Map<String, Uint8List> wrapped = example.scripts;
+  Map<String, Script> get scripts {
+    Map<String, Script> wrapped = example.scripts;
 
-    var scriptFiles = <String, Uint8List>{};
+    Map<String, Script> scriptFiles = <String, Script>{};
     scriptFiles.addAll(wrapped);
 
     // Create a new main file
@@ -677,21 +693,35 @@ class PartWrapExample extends Example {
       throw "Framework failure: "
           "Wanted to create wrapper file, but the file already exists!";
     }
-    scriptFiles[mainFilename] = new Uint8List.fromList(utf8.encode("""
-      part "${example.mainFilename}";
-    """));
+    Script originalMainScript = scriptFiles[example.mainFilename];
+    String preamble = originalMainScript.preamble;
+    scriptFiles[mainFilename] = new Script.fromSource("""
+${preamble}part "${example.mainFilename}";
+    """);
 
     // Modify the original main file to be part of the wrapper and add lots of
     // gunk so every actual position in the file is not a valid position in the
     // wrapper.
-    scriptFiles[example.mainFilename] = new Uint8List.fromList(utf8.encode("""
-      part of "${mainFilename}";
-      // La la la la la la la la la la la la la.
-      // La la la la la la la la la la la la la.
-      // La la la la la la la la la la la la la.
-      // La la la la la la la la la la la la la.
-      // La la la la la la la la la la la la la.
-    """) + scriptFiles[example.mainFilename]);
+    String originalMainSource = originalMainScript.sourceWithoutPreamble;
+    String partPrefix = """
+${preamble}part of "${mainFilename}";
+// La la la la la la la la la la la la la.
+// La la la la la la la la la la la la la.
+// La la la la la la la la la la la la la.
+// La la la la la la la la la la la la la.
+// La la la la la la la la la la la la la.
+
+""";
+    if (originalMainSource != null) {
+      scriptFiles[example.mainFilename] =
+          new Script.fromSource('$partPrefix$originalMainSource');
+    } else {
+      scriptFiles[example.mainFilename] = new Script(
+          new Uint8List.fromList(
+              utf8.encode(partPrefix) + originalMainScript.bytes),
+          '',
+          null);
+    }
 
     return scriptFiles;
   }
@@ -723,9 +753,9 @@ class Compile extends Step<Example, Null, MessageTestSuite> {
   Future<Result<Null>> run(Example example, MessageTestSuite suite) async {
     if (example == null) return pass(null);
     String dir = "${example.expectedCode}/${example.name}";
-    example.scripts.forEach((String fileName, Uint8List bytes) {
+    example.scripts.forEach((String fileName, Script script) {
       Uri uri = suite.fileSystem.currentDirectory.resolve("$dir/$fileName");
-      suite.fileSystem.entityForUri(uri).writeAsBytesSync(bytes);
+      suite.fileSystem.entityForUri(uri).writeAsBytesSync(script.bytes);
     });
     Uri main = suite.fileSystem.currentDirectory
         .resolve("$dir/${example.mainFilename}");
@@ -749,6 +779,7 @@ class Compile extends Step<Example, Null, MessageTestSuite> {
         example.configuration.apply(new CompilerOptions()
           ..sdkSummary = computePlatformBinariesLocation(forceBuildDir: true)
               .resolve("vm_platform_strong.dill")
+          ..explicitExperimentalFlags = example.experimentalFlags ?? {}
           ..target = new VmTarget(new TargetFlags())
           ..fileSystem = new HybridFileSystem(suite.fileSystem)
           ..packagesFileUri = dotPackagesUri
@@ -814,5 +845,29 @@ String relativize(Uri uri) {
   }
 }
 
+class Script {
+  final Uint8List bytes;
+  final String preamble;
+  final String sourceWithoutPreamble;
+
+  Script(this.bytes, this.preamble, this.sourceWithoutPreamble);
+
+  factory Script.fromSource(String source) {
+    List<String> lines = source.split('\n');
+    String firstLine = lines.first;
+    String preamble;
+    String sourceWithoutPreamble;
+    if (firstLine.trim().startsWith('//') && firstLine.contains('@dart=')) {
+      preamble = '$firstLine\n';
+      sourceWithoutPreamble = lines.skip(1).join('\n');
+    } else {
+      preamble = '';
+      sourceWithoutPreamble = source;
+    }
+    return new Script(new Uint8List.fromList(utf8.encode(source)), preamble,
+        sourceWithoutPreamble);
+  }
+}
+
 main([List<String> arguments = const []]) =>
     runMe(arguments, createContext, configurationPath: "../../testing.json");
diff --git a/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.expect b/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.expect
index ffd4dfea483..fb38c297d46 100644
--- a/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.expect
+++ b/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.expect
@@ -38,10 +38,6 @@ library /*isNonNullableByDefault*/;
 //   Constraints get constraints {}
 //   ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/crashes/crash_01/main_lib.dart:2:19: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-//   Constraints get constraints {}
-//                   ^
-//
 import self as mai;
 import "dart:core" as core;
 
@@ -49,11 +45,7 @@ abstract class RenderObject extends core::Object {
   synthetic constructor •() → mai::RenderObject
     : super core::Object::•()
     ;
-  get constraints() → invalid-type {
-    return let final Never #t1 = invalid-expression "pkg/front_end/testcases/general/crashes/crash_01/main_lib.dart:2:19: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-  Constraints get constraints {}
-                  ^" in null;
-  }
+  get constraints() → invalid-type {}
 }
 abstract class RenderObjectWithChildMixin extends mai::RenderObject /*isMixinDeclaration*/  {
 }
diff --git a/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.transformed.expect b/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.transformed.expect
index 28258584b2a..e5818cb2fc7 100644
--- a/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/general/crashes/crash_01/main.dart.weak.transformed.expect
@@ -38,10 +38,6 @@ library /*isNonNullableByDefault*/;
 //   Constraints get constraints {}
 //   ^^^^^^^^^^^
 //
-// pkg/front_end/testcases/general/crashes/crash_01/main_lib.dart:2:19: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-//   Constraints get constraints {}
-//                   ^
-//
 import self as mai;
 import "dart:core" as core;
 
@@ -49,11 +45,7 @@ abstract class RenderObject extends core::Object {
   synthetic constructor •() → mai::RenderObject
     : super core::Object::•()
     ;
-  get constraints() → invalid-type {
-    return let final Never #t1 = invalid-expression "pkg/front_end/testcases/general/crashes/crash_01/main_lib.dart:2:19: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-  Constraints get constraints {}
-                  ^" in null;
-  }
+  get constraints() → invalid-type {}
 }
 abstract class RenderObjectWithChildMixin extends mai::RenderObject /*isMixinDeclaration*/  {
 }
diff --git a/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.expect b/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.expect
index 922fc9e97f9..ac2becac763 100644
--- a/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.expect
+++ b/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.expect
@@ -20,10 +20,6 @@ library /*isNonNullableByDefault*/;
 //     co <{
 //     ^^
 //
-// pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart:2:8: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-//     co <{
-//        ^
-//
 import self as self;
 import "dart:core" as core;
 
@@ -31,9 +27,5 @@ class A extends core::Object {
   synthetic constructor •() → self::A
     : super core::Object::•()
     ;
-  operator <() → invalid-type {
-    return let final Never #t1 = invalid-expression "pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart:2:8: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-    co <{
-       ^" in null;
-  }
+  operator <() → invalid-type {}
 }
diff --git a/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.transformed.expect b/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.transformed.expect
index 922fc9e97f9..ac2becac763 100644
--- a/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart.weak.transformed.expect
@@ -20,10 +20,6 @@ library /*isNonNullableByDefault*/;
 //     co <{
 //     ^^
 //
-// pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart:2:8: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-//     co <{
-//        ^
-//
 import self as self;
 import "dart:core" as core;
 
@@ -31,9 +27,5 @@ class A extends core::Object {
   synthetic constructor •() → self::A
     : super core::Object::•()
     ;
-  operator <() → invalid-type {
-    return let final Never #t1 = invalid-expression "pkg/front_end/testcases/general/error_recovery/issue_39026.crash_dart:2:8: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-    co <{
-       ^" in null;
-  }
+  operator <() → invalid-type {}
 }
diff --git a/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.expect b/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.expect
index a495d01a7a0..65bece2ed6c 100644
--- a/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.expect
+++ b/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.expect
@@ -15,10 +15,6 @@ library /*isNonNullableByDefault*/;
 //     co operator <{
 //     ^^
 //
-// pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart:2:17: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-//     co operator <{
-//                 ^
-//
 import self as self;
 import "dart:core" as core;
 
@@ -26,9 +22,5 @@ class A extends core::Object {
   synthetic constructor •() → self::A
     : super core::Object::•()
     ;
-  operator <() → invalid-type {
-    return let final Never #t1 = invalid-expression "pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart:2:17: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-    co operator <{
-                ^" in null;
-  }
+  operator <() → invalid-type {}
 }
diff --git a/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.transformed.expect b/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.transformed.expect
index a495d01a7a0..65bece2ed6c 100644
--- a/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.transformed.expect
+++ b/pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart.weak.transformed.expect
@@ -15,10 +15,6 @@ library /*isNonNullableByDefault*/;
 //     co operator <{
 //     ^^
 //
-// pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart:2:17: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-//     co operator <{
-//                 ^
-//
 import self as self;
 import "dart:core" as core;
 
@@ -26,9 +22,5 @@ class A extends core::Object {
   synthetic constructor •() → self::A
     : super core::Object::•()
     ;
-  operator <() → invalid-type {
-    return let final Never #t1 = invalid-expression "pkg/front_end/testcases/general/error_recovery/issue_39026_prime.crash_dart:2:17: Error: A non-null value must be returned since the return type 'invalid-type' doesn't allow null.
-    co operator <{
-                ^" in null;
-  }
+  operator <() → invalid-type {}
 }
diff --git a/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.expect b/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.expect
index 1a240463251..459497af294 100644
Binary files a/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.expect and b/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.expect differ
diff --git a/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.transformed.expect b/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.transformed.expect
index 1a240463251..459497af294 100644
Binary files a/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.transformed.expect and b/pkg/front_end/testcases/regress/utf_16_le_content.crash_dart.weak.transformed.expect differ