diff --git a/.dart_tool/package_config.json b/.dart_tool/package_config.json
index 13301b65324..faad814af20 100644
--- a/.dart_tool/package_config.json
+++ b/.dart_tool/package_config.json
@@ -11,7 +11,7 @@
     "constraint, update this by running tools/generate_package_config.dart."
   ],
   "configVersion": 2,
-  "generated": "2021-07-26T14:57:34.624319",
+  "generated": "2021-07-27T19:27:52.638315",
   "generator": "tools/generate_package_config.dart",
   "packages": [
     {
@@ -725,7 +725,7 @@
       "name": "testing",
       "rootUri": "../pkg/testing",
       "packageUri": "lib/",
-      "languageVersion": "2.0"
+      "languageVersion": "2.12"
     },
     {
       "name": "typed_data",
diff --git a/pkg/testing/lib/src/analyze.dart b/pkg/testing/lib/src/analyze.dart
index 099d020c453..05009ecbe93 100644
--- a/pkg/testing/lib/src/analyze.dart
+++ b/pkg/testing/lib/src/analyze.dart
@@ -18,21 +18,21 @@ import 'log.dart' show isVerbose, splitLines;
 import 'suite.dart' show Suite;
 
 class Analyze extends Suite {
-  final Uri analysisOptions;
+  final Uri? analysisOptions;
 
   final List<Uri> uris;
 
   final List<RegExp> exclude;
 
-  final List<String> gitGrepPathspecs;
+  final List<String>? gitGrepPathspecs;
 
-  final List<String> gitGrepPatterns;
+  final List<String>? gitGrepPatterns;
 
   Analyze(this.analysisOptions, this.uris, this.exclude, this.gitGrepPathspecs,
       this.gitGrepPatterns)
       : super("analyze", "analyze", null);
 
-  Future<Null> run(Uri packages, List<Uri> extraUris) {
+  Future<Null> run(Uri packages, List<Uri>? extraUris) {
     List<Uri> allUris = new List<Uri>.from(uris);
     if (extraUris != null) {
       allUris.addAll(extraUris);
@@ -43,8 +43,8 @@ class Analyze extends Suite {
 
   static Future<Analyze> fromJsonMap(
       Uri base, Map json, List<Suite> suites) async {
-    String optionsPath = json["options"];
-    Uri optionsUri = optionsPath == null ? null : base.resolve(optionsPath);
+    String? optionsPath = json["options"];
+    Uri? optionsUri = optionsPath == null ? null : base.resolve(optionsPath);
 
     List<Uri> uris = json["uris"].map<Uri>((relative) {
       String r = relative;
@@ -54,9 +54,9 @@ class Analyze extends Suite {
     List<RegExp> exclude =
         json["exclude"].map<RegExp>((p) => new RegExp(p)).toList();
 
-    Map gitGrep = json["git grep"];
-    List<String> gitGrepPathspecs;
-    List<String> gitGrepPatterns;
+    Map? gitGrep = json["git grep"];
+    List<String>? gitGrepPathspecs;
+    List<String>? gitGrepPatterns;
     if (gitGrep != null) {
       gitGrepPathspecs = gitGrep["pathspecs"] == null
           ? const <String>["."]
@@ -73,13 +73,13 @@ class Analyze extends Suite {
 }
 
 class AnalyzerDiagnostic {
-  final String kind;
+  final String? kind;
 
-  final String detailedKind;
+  final String? detailedKind;
 
-  final String code;
+  final String? code;
 
-  final Uri uri;
+  final Uri? uri;
 
   final int line;
 
@@ -106,7 +106,7 @@ class AnalyzerDiagnostic {
     addPart() {
       parts.add(line
           .substring(start, index == -1 ? null : index)
-          .replaceAllMapped(unescapePattern, (Match m) => m[1]));
+          .replaceAllMapped(unescapePattern, (Match m) => m[1]!));
     }
 
     while (index != -1) {
@@ -136,8 +136,8 @@ class AnalyzerDiagnostic {
   String toString() {
     return kind == null
         ? "Malformed output from dartanalyzer:\n$message"
-        : "${uri.toFilePath()}:$line:$startColumn: "
-            "${kind == 'INFO' ? 'warning: hint' : kind.toLowerCase()}:\n"
+        : "${uri!.toFilePath()}:$line:$startColumn: "
+            "${kind == 'INFO' ? 'warning: hint' : kind!.toLowerCase()}:\n"
             "[$code] $message";
   }
 }
@@ -154,12 +154,12 @@ Stream<AnalyzerDiagnostic> parseAnalyzerOutput(
 
 /// Run dartanalyzer on all tests in [uris].
 Future<Null> analyzeUris(
-    Uri analysisOptions,
+    Uri? analysisOptions,
     Uri packages,
     List<Uri> uris,
     List<RegExp> exclude,
-    List<String> gitGrepPathspecs,
-    List<String> gitGrepPatterns) async {
+    List<String>? gitGrepPathspecs,
+    List<String>? gitGrepPatterns) async {
   if (uris.isEmpty) return;
   String topLevel;
   try {
@@ -205,7 +205,7 @@ Future<Null> analyzeUris(
     arguments.addAll(
         gitGrepPatterns.expand((String pattern) => <String>["-e", pattern]));
     arguments.add("--");
-    arguments.addAll(gitGrepPathspecs);
+    arguments.addAll(gitGrepPathspecs!);
     filesToAnalyze.addAll(splitLines(await git("grep", arguments))
         .map((String line) => line.trimRight()));
   }
@@ -245,7 +245,7 @@ Future<Null> analyzeUris(
   processAnalyzerOutput(Stream<AnalyzerDiagnostic> diagnostics) async {
     await for (AnalyzerDiagnostic diagnostic in diagnostics) {
       if (diagnostic.uri != null) {
-        String path = toFilePath(diagnostic.uri);
+        String path = toFilePath(diagnostic.uri!);
         if (!filesToAnalyze.contains(path)) continue;
       }
       String message = "$diagnostic";
@@ -283,7 +283,7 @@ String _findSdkPath() {
 }
 
 Future<String> git(String command, Iterable<String> arguments,
-    {String workingDirectory}) async {
+    {String? workingDirectory}) async {
   ProcessResult result = await Process.run(
       Platform.isWindows ? "git.bat" : "git",
       <String>[command]..addAll(arguments),
diff --git a/pkg/testing/lib/src/chain.dart b/pkg/testing/lib/src/chain.dart
index c56fc216f60..8ab797bcacb 100644
--- a/pkg/testing/lib/src/chain.dart
+++ b/pkg/testing/lib/src/chain.dart
@@ -115,7 +115,7 @@ abstract class ChainContext {
         .map((s) => s.substring(0, s.length - 3))
         .toList();
     TestExpectations expectations = await ReadTestExpectations(
-        <String>[suite.statusFile.toFilePath()], {}, expectationSet);
+        <String>[suite.statusFile!.toFilePath()], {}, expectationSet);
     Stream<TestDescription> stream = list(suite);
     if (suite.processMultitests) {
       stream = stream.transform(new MultitestTransformer());
@@ -149,12 +149,12 @@ abstract class ChainContext {
       final Set<Expectation> expectedOutcomes = processExpectedOutcomes(
           expectations.expectations(description.shortName), description);
       final StringBuffer sb = new StringBuffer();
-      final Step lastStep = steps.isNotEmpty ? steps.last : null;
+      final Step? lastStep = steps.isNotEmpty ? steps.last : null;
       final Iterator<Step> iterator = steps.iterator;
 
-      Result result;
+      Result? result;
       // Records the outcome of the last step that was run.
-      Step lastStepRun;
+      Step? lastStepRun;
 
       /// Performs one step of [iterator].
       ///
@@ -194,30 +194,30 @@ abstract class ChainContext {
           future = new Future.value(null);
         }
         future = future.then((_currentResult) async {
-          Result currentResult = _currentResult;
+          Result? currentResult = _currentResult;
           if (currentResult != null) {
             logger.logStepComplete(completed, unexpectedResults.length,
-                descriptions.length, suite, description, lastStepRun);
+                descriptions.length, suite, description, lastStepRun!);
             result = currentResult;
             if (currentResult.outcome == Expectation.Pass) {
               // The input to the next step is the output of this step.
-              return doStep(result.output);
+              return doStep(result!.output);
             }
           }
-          await cleanUp(description, result);
+          await cleanUp(description, result!);
           result =
-              processTestResult(description, result, lastStep == lastStepRun);
-          if (!expectedOutcomes.contains(result.outcome) &&
-              !expectedOutcomes.contains(result.outcome.canonical)) {
-            result.addLog("$sb");
-            unexpectedResults[description] = result;
+              processTestResult(description, result!, lastStep == lastStepRun);
+          if (!expectedOutcomes.contains(result!.outcome) &&
+              !expectedOutcomes.contains(result!.outcome.canonical)) {
+            result!.addLog("$sb");
+            unexpectedResults[description] = result!;
             unexpectedOutcomes[description] = expectedOutcomes;
             logger.logUnexpectedResult(
-                suite, description, result, expectedOutcomes);
+                suite, description, result!, expectedOutcomes);
             exitCode = 1;
           } else {
             logger.logExpectedResult(
-                suite, description, result, expectedOutcomes);
+                suite, description, result!, expectedOutcomes);
             logger.logMessage(sb);
           }
           logger.logTestComplete(++completed, unexpectedResults.length,
@@ -241,7 +241,7 @@ abstract class ChainContext {
     if (unexpectedResults.isNotEmpty) {
       unexpectedResults.forEach((TestDescription description, Result result) {
         logger.logUnexpectedResult(
-            suite, description, result, unexpectedOutcomes[description]);
+            suite, description, result, unexpectedOutcomes[description]!);
       });
       print("${unexpectedResults.length} failed:");
       unexpectedResults.forEach((TestDescription description, Result result) {
@@ -278,7 +278,7 @@ abstract class ChainContext {
       TestDescription description, Result result, bool last) {
     if (description is FileBasedTestDescription &&
         description.multitestExpectations != null) {
-      if (isError(description.multitestExpectations)) {
+      if (isError(description.multitestExpectations!)) {
         result =
             toNegativeTestResult(result, description.multitestExpectations);
       }
@@ -286,14 +286,14 @@ abstract class ChainContext {
       if (result.outcome == Expectation.Pass) {
         result.addLog("Negative test didn't report an error.\n");
       } else if (result.outcome == Expectation.Fail) {
-        result.addLog("Negative test reported an error as expeceted.\n");
+        result.addLog("Negative test reported an error as expected.\n");
       }
       result = toNegativeTestResult(result);
     }
     return result;
   }
 
-  Result toNegativeTestResult(Result result, [Set<String> expectations]) {
+  Result toNegativeTestResult(Result result, [Set<String>? expectations]) {
     Expectation outcome = result.outcome;
     if (outcome == Expectation.Pass) {
       if (expectations == null) {
@@ -312,9 +312,9 @@ abstract class ChainContext {
     return result.copyWithOutcome(outcome);
   }
 
-  Future<void> cleanUp(TestDescription description, Result result) => null;
+  Future<void> cleanUp(TestDescription description, Result result) async {}
 
-  Future<void> postRun() => null;
+  Future<void> postRun() async {}
 }
 
 abstract class Step<I, O, C extends ChainContext> {
@@ -355,25 +355,25 @@ abstract class Step<I, O, C extends ChainContext> {
 
   Result<O> crash(error, StackTrace trace) => new Result<O>.crash(error, trace);
 
-  Result<O> fail(O output, [error, StackTrace trace]) {
+  Result<O> fail(O output, [error, StackTrace? trace]) {
     return new Result<O>.fail(output, error, trace);
   }
 }
 
 class Result<O> {
-  final O output;
+  final O? output;
 
   final Expectation outcome;
 
   final error;
 
-  final StackTrace trace;
+  final StackTrace? trace;
 
   final List<String> logs = <String>[];
 
   /// If set, running the test with '-D$autoFixCommand' will automatically
   /// update the test to match new expectations.
-  final String autoFixCommand;
+  final String? autoFixCommand;
 
   /// If set, the test can be fixed by running
   ///
@@ -391,7 +391,7 @@ class Result<O> {
   Result.crash(error, StackTrace trace)
       : this(null, Expectation.Crash, error, trace: trace);
 
-  Result.fail(O output, [error, StackTrace trace])
+  Result.fail(O output, [error, StackTrace? trace])
       : this(output, Expectation.Fail, error, trace: trace);
 
   bool get isPass => outcome == Expectation.Pass;
@@ -420,7 +420,8 @@ class Result<O> {
 Future<Null> runChain(CreateContext f, Map<String, String> environment,
     Set<String> selectors, String jsonText) {
   return withErrorHandling(() async {
-    Chain suite = new Suite.fromJsonMap(Uri.base, json.decode(jsonText));
+    Chain suite =
+        new Suite.fromJsonMap(Uri.base, json.decode(jsonText)) as Chain;
     print("Running ${suite.name}");
     ChainContext context = await f(suite, environment);
     return context.run(suite, selectors);
diff --git a/pkg/testing/lib/src/discover.dart b/pkg/testing/lib/src/discover.dart
index 3fd09e996dd..6b761abb90f 100644
--- a/pkg/testing/lib/src/discover.dart
+++ b/pkg/testing/lib/src/discover.dart
@@ -20,10 +20,10 @@ List<String> get dartArguments =>
     <String>["-c", "--packages=${packageConfig.toFilePath()}"];
 
 Stream<FileBasedTestDescription> listTests(List<Uri> testRoots,
-    {Pattern pattern}) {
+    {Pattern? pattern}) {
   StreamController<FileBasedTestDescription> controller =
       new StreamController<FileBasedTestDescription>();
-  Map<Uri, StreamSubscription> subscriptions = <Uri, StreamSubscription>{};
+  Map<Uri, StreamSubscription?> subscriptions = <Uri, StreamSubscription>{};
   for (Uri testRootUri in testRoots) {
     subscriptions[testRootUri] = null;
     Directory testRoot = new Directory.fromUri(testRootUri);
@@ -32,8 +32,9 @@ Stream<FileBasedTestDescription> listTests(List<Uri> testRoots,
         Stream<FileSystemEntity> stream =
             testRoot.list(recursive: true, followLinks: false);
         var subscription = stream.listen((FileSystemEntity entity) {
-          FileBasedTestDescription description = FileBasedTestDescription
-              .from(testRootUri, entity, pattern: pattern);
+          FileBasedTestDescription? description = FileBasedTestDescription.from(
+              testRootUri, entity,
+              pattern: pattern);
           if (description != null) {
             controller.add(description);
           }
@@ -59,7 +60,7 @@ Stream<FileBasedTestDescription> listTests(List<Uri> testRoots,
 }
 
 Uri computePackageConfig() {
-  String path = Platform.packageConfig;
+  String? path = Platform.packageConfig;
   if (path != null) return Uri.base.resolve(path);
   return Uri.base.resolve(".packages");
 }
@@ -72,7 +73,7 @@ const _dartSdk = (String.fromEnvironment("DART_SDK", defaultValue: "1") ==
     : null;
 
 Uri computeDartSdk() {
-  String dartSdkPath = Platform.environment["DART_SDK"] ?? _dartSdk;
+  String? dartSdkPath = Platform.environment["DART_SDK"] ?? _dartSdk;
   if (dartSdkPath != null) {
     return Uri.base.resolveUri(new Uri.file(dartSdkPath));
   } else {
@@ -83,7 +84,7 @@ Uri computeDartSdk() {
 }
 
 Future<Process> startDart(Uri program,
-    [List<String> arguments, List<String> vmArguments]) {
+    [List<String>? arguments, List<String>? vmArguments]) {
   List<String> allArguments = <String>[];
   allArguments.addAll(vmArguments ?? dartArguments);
   allArguments.add(program.toFilePath());
diff --git a/pkg/testing/lib/src/error_handling.dart b/pkg/testing/lib/src/error_handling.dart
index 452e4b72f6c..872f2f0ca84 100644
--- a/pkg/testing/lib/src/error_handling.dart
+++ b/pkg/testing/lib/src/error_handling.dart
@@ -12,13 +12,14 @@ import 'dart:isolate' show ReceivePort;
 
 import 'log.dart';
 
-Future<T> withErrorHandling<T>(Future<T> f(), {Logger logger}) async {
+Future<T?> withErrorHandling<T>(Future<T> f(), {Logger? logger}) async {
   final ReceivePort port = new ReceivePort();
   try {
     return await f();
   } catch (e, trace) {
     exitCode = 1;
     stderr.writeln(e);
+    // ignore: unnecessary_null_comparison
     if (trace != null) {
       stderr.writeln(trace);
     }
diff --git a/pkg/testing/lib/src/expectation.dart b/pkg/testing/lib/src/expectation.dart
index 58efce8268b..153adee7855 100644
--- a/pkg/testing/lib/src/expectation.dart
+++ b/pkg/testing/lib/src/expectation.dart
@@ -42,7 +42,7 @@ class Expectation {
 
   String toString() => name;
 
-  static Expectation fromGroup(ExpectationGroup group) {
+  static Expectation? fromGroup(ExpectationGroup group) {
     switch (group) {
       case ExpectationGroup.Crash:
         return Expectation.Crash;
@@ -57,7 +57,6 @@ class Expectation {
       case ExpectationGroup.Timeout:
         return Expectation.Timeout;
     }
-    throw "Unhandled group: '$group'.";
   }
 }
 
@@ -89,8 +88,8 @@ class ExpectationSet {
     Map<String, Expectation> internalMap =
         new Map<String, Expectation>.from(Default.internalMap);
     for (Map map in data) {
-      String name;
-      String group;
+      String? name;
+      String? group;
       map.forEach((_key, _value) {
         String key = _key;
         String value = _value;
@@ -113,12 +112,12 @@ class ExpectationSet {
       if (group == null) {
         throw "No group provided in '$map'";
       }
-      Expectation expectation = new Expectation(name, groupFromString(group));
-      name = name.toLowerCase();
+      Expectation expectation = new Expectation(name!, groupFromString(group!));
+      name = name!.toLowerCase();
       if (internalMap.containsKey(name)) {
         throw "Duplicated expectation name: '$name'.";
       }
-      internalMap[name] = expectation;
+      internalMap[name!] = expectation;
     }
     return new ExpectationSet(internalMap);
   }
diff --git a/pkg/testing/lib/src/log.dart b/pkg/testing/lib/src/log.dart
index 7e6fafd0f71..d46ab0a3353 100644
--- a/pkg/testing/lib/src/log.dart
+++ b/pkg/testing/lib/src/log.dart
@@ -78,19 +78,19 @@ abstract class Logger {
 class StdoutLogger implements Logger {
   const StdoutLogger();
 
-  void logTestStart(int completed, int failed, int total, Suite suite,
-      TestDescription description) {}
+  void logTestStart(int completed, int failed, int total, Suite? suite,
+      TestDescription? description) {}
 
-  void logTestComplete(int completed, int failed, int total, Suite suite,
-      TestDescription description) {
+  void logTestComplete(int completed, int failed, int total, Suite? suite,
+      TestDescription? description) {
     String message = formatProgress(completed, failed, total);
     if (suite != null) {
-      message += ": ${formatTestDescription(suite, description)}";
+      message += ": ${formatTestDescription(suite, description!)}";
     }
     logProgress(message);
   }
 
-  void logStepStart(int completed, int failed, int total, Suite suite,
+  void logStepStart(int completed, int failed, int total, Suite? suite,
       TestDescription description, Step step) {
     String message = formatProgress(completed, failed, total);
     if (suite != null) {
@@ -102,7 +102,7 @@ class StdoutLogger implements Logger {
     logProgress(message);
   }
 
-  void logStepComplete(int completed, int failed, int total, Suite suite,
+  void logStepComplete(int completed, int failed, int total, Suite? suite,
       TestDescription description, Step step) {
     if (!step.isAsync) return;
     String message = formatProgress(completed, failed, total);
@@ -152,7 +152,7 @@ class StdoutLogger implements Logger {
   void logUnexpectedResult(Suite suite, TestDescription description,
       Result result, Set<Expectation> expectedOutcomes) {
     print("${eraseLine}UNEXPECTED: ${suite.name}/${description.shortName}");
-    Uri statusFile = suite.statusFile;
+    Uri? statusFile = suite.statusFile;
     if (statusFile != null) {
       String path = statusFile.toFilePath();
       if (result.outcome == Expectation.Pass) {
@@ -186,6 +186,7 @@ class StdoutLogger implements Logger {
 
   void logUncaughtError(error, StackTrace stackTrace) {
     logMessage(error);
+    // ignore: unnecessary_null_comparison
     if (stackTrace != null) {
       logMessage(stackTrace);
     }
diff --git a/pkg/testing/lib/src/multitest.dart b/pkg/testing/lib/src/multitest.dart
index d87ac1bd3c6..62bb63e550d 100644
--- a/pkg/testing/lib/src/multitest.dart
+++ b/pkg/testing/lib/src/multitest.dart
@@ -51,8 +51,8 @@ class MultitestTransformer
 
     nextTest:
     await for (TestDescription test in stream) {
-      FileBasedTestDescription multitest;
-      String contents;
+      FileBasedTestDescription? multitest;
+      String? contents;
       if (test is FileBasedTestDescription) {
         contents = await test.file.readAsString();
         if (contents.contains(multitestMarker)) {
@@ -73,11 +73,11 @@ class MultitestTransformer
         "none": new Set<String>(),
       };
       int lineNumber = 0;
-      for (String line in splitLines(contents)) {
+      for (String line in splitLines(contents!)) {
         lineNumber++;
         int index = line.indexOf(multitestMarker);
-        String subtestName;
-        List<String> subtestOutcomesList;
+        String? subtestName;
+        List<String>? subtestOutcomesList;
         if (index != -1) {
           String annotationText =
               line.substring(index + _multitestMarkerLength).trim();
@@ -102,7 +102,7 @@ class MultitestTransformer
           lines.add(line);
           Set<String> subtestOutcomes =
               outcomes.putIfAbsent(subtestName, () => new Set<String>());
-          if (subtestOutcomesList.length != 1 ||
+          if (subtestOutcomesList!.length != 1 ||
               subtestOutcomesList.single != "continued") {
             for (String outcome in subtestOutcomesList) {
               if (validOutcomes.contains(outcome)) {
@@ -125,8 +125,9 @@ class MultitestTransformer
       Directory generated =
           new Directory.fromUri(root.resolve(multitest.shortName));
       generated = await generated.create(recursive: true);
-      for (String name in testsAsLines.keys) {
-        List<String> lines = testsAsLines[name];
+      for (MapEntry<String, List<String>> entry in testsAsLines.entries) {
+        String name = entry.key;
+        List<String> lines = entry.value;
         Uri uri = generated.uri.resolve("${name}_generated.dart");
         FileBasedTestDescription subtest =
             new FileBasedTestDescription(root, new File.fromUri(uri));
diff --git a/pkg/testing/lib/src/run.dart b/pkg/testing/lib/src/run.dart
index 7ce9bd02dbf..45672364477 100644
--- a/pkg/testing/lib/src/run.dart
+++ b/pkg/testing/lib/src/run.dart
@@ -34,10 +34,10 @@ import 'zone_helper.dart' show acknowledgeControlMessages;
 
 import 'run_tests.dart' show CommandLine;
 
-Future<TestRoot> computeTestRoot(String configurationPath, Uri base) {
+Future<TestRoot> computeTestRoot(String? configurationPath, Uri? base) {
   Uri configuration = configurationPath == null
       ? Uri.base.resolve("testing.json")
-      : base.resolve(configurationPath);
+      : base!.resolve(configurationPath);
   return TestRoot.fromUri(configuration);
 }
 
@@ -50,8 +50,8 @@ Future<TestRoot> computeTestRoot(String configurationPath, Uri base) {
 /// `testing.json` isn't located in the current working directory and is a path
 /// relative to [me] which defaults to `Platform.script`.
 Future<Null> runMe(List<String> arguments, CreateContext f,
-    {String configurationPath,
-    Uri me,
+    {String? configurationPath,
+    Uri? me,
     int shards = 1,
     int shard = 0,
     Logger logger: const StdoutLogger()}) {
@@ -98,7 +98,7 @@ Future<Null> runMe(List<String> arguments, CreateContext f,
 /// `testing.json` isn't located in the current working directory and is a path
 /// relative to `Uri.base`.
 Future<Null> run(List<String> arguments, List<String> suiteNames,
-    [String configurationPath]) {
+    [String? configurationPath]) {
   return withErrorHandling(() async {
     TestRoot root = await computeTestRoot(configurationPath, Uri.base);
     List<Suite> suites = root.suites
@@ -106,7 +106,7 @@ Future<Null> run(List<String> arguments, List<String> suiteNames,
         .toList();
     SuiteRunner runner = new SuiteRunner(suites, <String, String>{},
         const <String>[], new Set<String>(), new Set<String>());
-    String program = await runner.generateDartProgram();
+    String? program = await runner.generateDartProgram();
     await runner.analyze(root.packages);
     if (program != null) {
       await runProgram(program, root.packages);
@@ -125,7 +125,7 @@ Future<Null> runProgram(String program, Uri packages) async {
       errorsAreFatal: false,
       checked: true,
       packageConfig: packages);
-  List error;
+  List? error;
   var subscription = isolate.errors.listen((data) {
     error = data;
     exitPort.close();
@@ -137,7 +137,7 @@ Future<Null> runProgram(String program, Uri packages) async {
   subscription.cancel();
   return error == null
       ? null
-      : new Future<Null>.error(error[0], new StackTrace.fromString(error[1]));
+      : new Future<Null>.error(error![0], new StackTrace.fromString(error![1]));
 }
 
 class SuiteRunner {
@@ -162,7 +162,7 @@ class SuiteRunner {
         (selectedSuites.isEmpty || selectedSuites.contains(suite.name));
   }
 
-  Future<String> generateDartProgram() async {
+  Future<String?> generateDartProgram() async {
     testUris.clear();
     StringBuffer imports = new StringBuffer();
     StringBuffer dart = new StringBuffer();
@@ -238,10 +238,10 @@ Future<Null> main() async {
   }
 
   Stream<FileBasedTestDescription> listDescriptions() async* {
-    for (Dart suite in suites.where((Suite suite) => suite is Dart)) {
+    for (Dart suite in suites.whereType<Dart>()) {
       await for (FileBasedTestDescription description
           in listTests(<Uri>[suite.uri], pattern: "")) {
-        testUris.add(await Isolate.resolvePackageUri(description.uri));
+        testUris.add((await Isolate.resolvePackageUri(description.uri))!);
         if (shouldRunSuite(suite)) {
           String path = description.file.uri.path;
           if (suite.exclude.any((RegExp r) => path.contains(r))) continue;
@@ -254,19 +254,19 @@ Future<Null> main() async {
   }
 
   Stream<Chain> listChainSuites() async* {
-    for (Chain suite in suites.where((Suite suite) => suite is Chain)) {
-      testUris.add(await Isolate.resolvePackageUri(suite.source));
+    for (Chain suite in suites.whereType<Chain>()) {
+      testUris.add((await Isolate.resolvePackageUri(suite.source))!);
       if (shouldRunSuite(suite)) {
         yield suite;
       }
     }
   }
 
-  Iterable<Suite> listTestDartSuites() {
-    return suites.where((Suite suite) => suite is TestDart);
+  Iterable<TestDart> listTestDartSuites() {
+    return suites.whereType<TestDart>();
   }
 
-  Iterable<Suite> listAnalyzerSuites() {
-    return suites.where((Suite suite) => suite is Analyze);
+  Iterable<Analyze> listAnalyzerSuites() {
+    return suites.whereType<Analyze>();
   }
 }
diff --git a/pkg/testing/lib/src/run_tests.dart b/pkg/testing/lib/src/run_tests.dart
index b4ccd3be64a..03d5d3de4fc 100644
--- a/pkg/testing/lib/src/run_tests.dart
+++ b/pkg/testing/lib/src/run_tests.dart
@@ -64,7 +64,7 @@ class CommandLine {
 
   Iterable<String> get selectors => arguments;
 
-  Future<Uri> get configuration async {
+  Future<Uri?> get configuration async {
     const String configPrefix = "--config=";
     List<String> configurationPaths = options
         .where((String option) => option.startsWith(configPrefix))
@@ -111,7 +111,7 @@ class CommandLine {
     }
     const StdoutLogger()
         .logMessage("Reading configuration file '$configurationPath'.");
-    Uri configuration =
+    Uri? configuration =
         await Isolate.resolvePackageUri(Uri.base.resolve(configurationPath));
     if (configuration == null ||
         !await new File.fromUri(configuration).exists()) {
@@ -147,7 +147,7 @@ main(List<String> arguments) => withErrorHandling(() async {
         enableVerboseOutput();
       }
       Map<String, String> environment = cl.environment;
-      Uri configuration = await cl.configuration;
+      Uri? configuration = await cl.configuration;
       if (configuration == null) return;
       if (!isVerbose) {
         print("Use --verbose to display more details.");
@@ -155,7 +155,7 @@ main(List<String> arguments) => withErrorHandling(() async {
       TestRoot root = await TestRoot.fromUri(configuration);
       SuiteRunner runner = new SuiteRunner(
           root.suites, environment, cl.selectors, cl.selectedSuites, cl.skip);
-      String program = await runner.generateDartProgram();
+      String? program = await runner.generateDartProgram();
       bool hasAnalyzerSuites = await runner.analyze(root.packages);
       Stopwatch sw = new Stopwatch()..start();
       if (program == null) {
@@ -178,7 +178,7 @@ Future<void> runTests(Map<String, Function> tests) =>
         try {
           await runGuarded(() {
             print("Running test $name");
-            return tests[name]();
+            return tests[name]!();
           }, printLineOnStdout: sb.writeln);
           const StdoutLogger().logMessage(sb);
         } catch (e) {
diff --git a/pkg/testing/lib/src/stdio_process.dart b/pkg/testing/lib/src/stdio_process.dart
index 16bbfdfdfb0..587a07f570d 100644
--- a/pkg/testing/lib/src/stdio_process.dart
+++ b/pkg/testing/lib/src/stdio_process.dart
@@ -41,13 +41,13 @@ class StdioProcess {
   }
 
   static Future<StdioProcess> run(String executable, List<String> arguments,
-      {String input,
-      Duration timeout: const Duration(seconds: 60),
+      {String? input,
+      Duration? timeout: const Duration(seconds: 60),
       bool suppressOutput: true,
       bool runInShell: false}) async {
     Process process =
         await Process.start(executable, arguments, runInShell: runInShell);
-    Timer timer;
+    Timer? timer;
     StringBuffer sb = new StringBuffer();
     if (timeout != null) {
       timer = new Timer(timeout, () {
@@ -75,8 +75,8 @@ class StdioProcess {
       stdoutStream = stdoutStream.transform(transformToStdio(io.stdout));
       stderrStream = stderrStream.transform(transformToStdio(io.stderr));
     }
-    Future<List<String>> stdoutFuture = stdoutStream.toList();
-    Future<List<String>> stderrFuture = stderrStream.toList();
+    Future<List<String>> stdoutFuture = stdoutStream.toList() as Future<List<String>>;
+    Future<List<String>> stderrFuture = stderrStream.toList() as Future<List<String>>;
     int exitCode = await process.exitCode;
     timer?.cancel();
     sb.writeAll(await stdoutFuture);
diff --git a/pkg/testing/lib/src/suite.dart b/pkg/testing/lib/src/suite.dart
index 54aa7c8ca5f..b1597971452 100644
--- a/pkg/testing/lib/src/suite.dart
+++ b/pkg/testing/lib/src/suite.dart
@@ -14,7 +14,7 @@ abstract class Suite {
 
   final String kind;
 
-  final Uri statusFile;
+  final Uri? statusFile;
 
   Suite(this.name, this.kind, this.statusFile);
 
diff --git a/pkg/testing/lib/src/test_dart/path.dart b/pkg/testing/lib/src/test_dart/path.dart
index 3c5fbeaa065..f0b5b5105ac 100644
--- a/pkg/testing/lib/src/test_dart/path.dart
+++ b/pkg/testing/lib/src/test_dart/path.dart
@@ -192,7 +192,7 @@ class Path {
   Path makeCanonical() {
     bool isAbs = isAbsolute;
     List segs = segments();
-    String drive;
+    String? drive;
     if (isAbs && !segs.isEmpty && segs[0].length == 2 && segs[0][1] == ':') {
       drive = segs[0];
       segs.removeRange(0, 1);
@@ -267,7 +267,7 @@ class Path {
   }
 
   List<String> segments() {
-    List result = _path.split('/');
+    List<String> result = _path.split('/');
     if (isAbsolute) result.removeRange(0, 1);
     if (hasTrailingSeparator) result.removeLast();
     return result;
diff --git a/pkg/testing/lib/src/test_dart/status_expression.dart b/pkg/testing/lib/src/test_dart/status_expression.dart
index 200d241725b..395483a6644 100644
--- a/pkg/testing/lib/src/test_dart/status_expression.dart
+++ b/pkg/testing/lib/src/test_dart/status_expression.dart
@@ -64,7 +64,9 @@ class Tokenizer {
     if (!testRegexp.hasMatch(expression)) {
       throw new FormatException("Syntax error in '$expression'");
     }
-    for (Match match in regexp.allMatches(expression)) tokens.add(match[0]);
+    for (Match match in regexp.allMatches(expression)) {
+      tokens.add(match[0]!);
+    }
     return tokens;
   }
 }
@@ -179,8 +181,8 @@ class SetConstant implements SetExpression {
 // An iterator that allows peeking at the current token.
 class Scanner {
   List<String> tokens;
-  Iterator tokenIterator;
-  String current;
+  late Iterator tokenIterator;
+  String? current;
 
   Scanner(this.tokens) {
     tokenIterator = tokens.iterator;
@@ -241,11 +243,11 @@ class ExpressionParser {
       scanner.advance();
       return value;
     }
-    if (!new RegExp(r"^\w+$").hasMatch(scanner.current)) {
+    if (!new RegExp(r"^\w+$").hasMatch(scanner.current!)) {
       throw new FormatException(
           "Expected identifier in expression, got ${scanner.current}");
     }
-    SetExpression value = new SetConstant(scanner.current);
+    SetExpression value = new SetConstant(scanner.current!);
     scanner.advance();
     return value;
   }
@@ -290,21 +292,21 @@ class ExpressionParser {
           "Expected \$ in expression, got ${scanner.current}");
     }
     scanner.advance();
-    if (!new RegExp(r"^\w+$").hasMatch(scanner.current)) {
+    if (!new RegExp(r"^\w+$").hasMatch(scanner.current!)) {
       throw new FormatException(
           "Expected identifier in expression, got ${scanner.current}");
     }
-    TermVariable left = new TermVariable(scanner.current);
+    TermVariable left = new TermVariable(scanner.current!);
     scanner.advance();
     if (scanner.current == Token.EQUALS ||
         scanner.current == Token.NOT_EQUALS) {
       bool negate = scanner.current == Token.NOT_EQUALS;
       scanner.advance();
-      if (!new RegExp(r"^\w+$").hasMatch(scanner.current)) {
+      if (!new RegExp(r"^\w+$").hasMatch(scanner.current!)) {
         throw new FormatException(
             "Expected value in expression, got ${scanner.current}");
       }
-      TermConstant right = new TermConstant(scanner.current);
+      TermConstant right = new TermConstant(scanner.current!);
       scanner.advance();
       return new Comparison(left, right, negate);
     } else {
diff --git a/pkg/testing/lib/src/test_dart/status_file_parser.dart b/pkg/testing/lib/src/test_dart/status_file_parser.dart
index 1ce711fbd50..a3d2f5887fc 100644
--- a/pkg/testing/lib/src/test_dart/status_file_parser.dart
+++ b/pkg/testing/lib/src/test_dart/status_file_parser.dart
@@ -29,7 +29,7 @@ class StatusFile {
 class Section {
   final StatusFile statusFile;
 
-  final BooleanExpression condition;
+  final BooleanExpression? condition;
   final List<TestRule> testRules;
   final int lineNumber;
 
@@ -40,7 +40,7 @@ class Section {
       : testRules = <TestRule>[];
 
   bool isEnabled(Map<String, String> environment) =>
-      condition == null || condition.evaluate(environment);
+      condition == null || condition!.evaluate(environment);
 
   String toString() {
     return "Section: $condition";
@@ -93,17 +93,17 @@ void ReadConfigurationInto(Path path, List<Section> sections, void onDone()) {
 
   lines.listen((String line) {
     lineNumber++;
-    Match match = SplitComment.firstMatch(line);
-    line = (match == null) ? "" : match[1];
+    Match? match = SplitComment.firstMatch(line);
+    line = (match == null) ? "" : match[1]!;
     line = line.trim();
     if (line.isEmpty) return;
 
     // Extract the comment to get the issue number if needed.
-    String comment = (match == null || match[2] == null) ? "" : match[2];
+    String comment = (match == null || match[2] == null) ? "" : match[2]!;
 
     match = HeaderPattern.firstMatch(line);
     if (match != null) {
-      String condition_string = match[1].trim();
+      String condition_string = match[1]!.trim();
       List<String> tokens = new Tokenizer(condition_string).tokenize();
       ExpressionParser parser = new ExpressionParser(new Scanner(tokens));
       currentSection =
@@ -114,21 +114,21 @@ void ReadConfigurationInto(Path path, List<Section> sections, void onDone()) {
 
     match = RulePattern.firstMatch(line);
     if (match != null) {
-      String name = match[1].trim();
+      String name = match[1]!.trim();
       // TODO(whesse): Handle test names ending in a wildcard (*).
-      String expression_string = match[2].trim();
+      String expression_string = match[2]!.trim();
       List<String> tokens = new Tokenizer(expression_string).tokenize();
       SetExpression expression =
           new ExpressionParser(new Scanner(tokens)).parseSetExpression();
 
       // Look for issue number in comment.
-      String issueString = null;
+      String? issueString = null;
       match = IssueNumberPattern.firstMatch(comment);
       if (match != null) {
         issueString = match[1];
         if (issueString == null) issueString = match[2];
       }
-      int issue = issueString != null ? int.parse(issueString) : null;
+      int? issue = issueString != null ? int.parse(issueString) : null;
       currentSection.testRules
           .add(new TestRule(name, expression, issue, lineNumber));
       return;
@@ -141,7 +141,7 @@ void ReadConfigurationInto(Path path, List<Section> sections, void onDone()) {
 class TestRule {
   String name;
   SetExpression expression;
-  int issue;
+  int? issue;
   int lineNumber;
 
   TestRule(this.name, this.expression, this.issue, this.lineNumber);
@@ -161,8 +161,8 @@ class TestExpectations {
 
   Map<String, Set<Expectation>> _map;
   bool _preprocessed = false;
-  Map<String, RegExp> _regExpCache;
-  Map<String, List<RegExp>> _keyToRegExps;
+  Map<String, RegExp>? _regExpCache;
+  Map<String, List<RegExp>>? _keyToRegExps;
 
   /**
    * Create a TestExpectations object. See the [expectations] method
@@ -203,7 +203,7 @@ class TestExpectations {
     _preprocessForMatching();
 
     _map.forEach((key, expectation) {
-      List regExps = _keyToRegExps[key];
+      List<RegExp> regExps = _keyToRegExps![key]!;
       if (regExps.length > splitFilename.length) return;
       for (var i = 0; i < regExps.length; i++) {
         if (!regExps[i].hasMatch(splitFilename[i])) return;
@@ -231,20 +231,19 @@ class TestExpectations {
     _regExpCache = {};
 
     _map.forEach((key, expectations) {
-      if (_keyToRegExps[key] != null) return;
+      if (_keyToRegExps![key] != null) return;
       var splitKey = key.split('/');
-      var regExps = new List<RegExp>.filled(splitKey.length, null);
-      for (var i = 0; i < splitKey.length; i++) {
+      var regExps = new List<RegExp>.generate(splitKey.length, (int i) {
         var component = splitKey[i];
-        var regExp = _regExpCache[component];
+        var regExp = _regExpCache![component];
         if (regExp == null) {
           var pattern = "^${splitKey[i]}\$".replaceAll('*', '.*');
           regExp = new RegExp(pattern);
-          _regExpCache[component] = regExp;
+          _regExpCache![component] = regExp;
         }
-        regExps[i] = regExp;
-      }
-      _keyToRegExps[key] = regExps;
+        return regExp;
+      }, growable: false);
+      _keyToRegExps![key] = regExps;
     });
 
     _regExpCache = null;
diff --git a/pkg/testing/lib/src/test_description.dart b/pkg/testing/lib/src/test_description.dart
index 7480ee0118e..1fe0ac15c19 100644
--- a/pkg/testing/lib/src/test_description.dart
+++ b/pkg/testing/lib/src/test_description.dart
@@ -17,11 +17,11 @@ abstract class TestDescription implements Comparable<TestDescription> {
 class FileBasedTestDescription extends TestDescription {
   final Uri root;
   final File file;
-  final Uri output;
+  final Uri? output;
 
   /// If non-null, this is a generated multitest, and the set contains the
   /// expected outcomes.
-  Set<String> multitestExpectations;
+  Set<String>? multitestExpectations;
 
   FileBasedTestDescription(this.root, this.file, {this.output});
 
@@ -52,8 +52,8 @@ class FileBasedTestDescription extends TestDescription {
     sink.writeln('.main,');
   }
 
-  static FileBasedTestDescription from(Uri root, FileSystemEntity entity,
-      {Pattern pattern}) {
+  static FileBasedTestDescription? from(Uri root, FileSystemEntity entity,
+      {Pattern? pattern}) {
     if (entity is! File) return null;
     pattern ??= "_test.dart";
     String path = entity.uri.path;
diff --git a/pkg/testing/lib/src/test_root.dart b/pkg/testing/lib/src/test_root.dart
index 9e3b65e7285..554abee9123 100644
--- a/pkg/testing/lib/src/test_root.dart
+++ b/pkg/testing/lib/src/test_root.dart
@@ -49,7 +49,7 @@ class TestRoot {
 
   TestRoot(this.packages, this.suites);
 
-  Analyze get analyze => suites.last;
+  Analyze get analyze => suites.last as Analyze;
 
   List<Uri> get urisToAnalyze => analyze.uris;
 
diff --git a/pkg/testing/lib/src/zone_helper.dart b/pkg/testing/lib/src/zone_helper.dart
index d8d7d397010..a378d7a430a 100644
--- a/pkg/testing/lib/src/zone_helper.dart
+++ b/pkg/testing/lib/src/zone_helper.dart
@@ -14,8 +14,8 @@ import 'dart:isolate' show Capability, Isolate, ReceivePort;
 import 'log.dart' show StdoutLogger;
 
 Future runGuarded(Future f(),
-    {void printLineOnStdout(line),
-    void handleLateError(error, StackTrace stackTrace)}) {
+    {void Function(String)? printLineOnStdout,
+    void Function(dynamic, StackTrace)? handleLateError}) {
   var printWrapper;
   if (printLineOnStdout != null) {
     printWrapper = (_1, _2, _3, String line) {
@@ -39,6 +39,7 @@ Future runGuarded(Future f(),
         // Ignored.
       }
       stderr
+          // ignore: unnecessary_null_comparison
           .write("$errorString\n" + (stackTrace == null ? "" : "$stackTrace"));
       stderr.flush();
       exit(255);
@@ -81,7 +82,7 @@ Future runGuarded(Future f(),
 /// Ping [isolate] to ensure control messages have been delivered.  Control
 /// messages are things like [Isolate.addErrorListener] and
 /// [Isolate.addOnExitListener].
-Future acknowledgeControlMessages(Isolate isolate, {Capability resume}) {
+Future acknowledgeControlMessages(Isolate isolate, {Capability? resume}) {
   ReceivePort ping = new ReceivePort();
   Isolate.current.ping(ping.sendPort);
   if (resume == null) {
diff --git a/pkg/testing/pubspec.yaml b/pkg/testing/pubspec.yaml
index f012946a943..d85f43fb05b 100644
--- a/pkg/testing/pubspec.yaml
+++ b/pkg/testing/pubspec.yaml
@@ -4,4 +4,4 @@ name: testing
 # This package is not intended for consumption on pub.dev. DO NOT publish.
 publish_to: none
 environment:
-  sdk: '>=2.0.0 <3.0.0'
+  sdk: '>=2.12.0 <3.0.0'