diff --git a/pkg/front_end/test/fasta/parser/type_info_test.dart b/pkg/front_end/test/fasta/parser/type_info_test.dart
index 26fb78f..8402482 100644
--- a/pkg/front_end/test/fasta/parser/type_info_test.dart
+++ b/pkg/front_end/test/fasta/parser/type_info_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:_fe_analyzer_shared/src/parser/parser.dart';
 import 'package:_fe_analyzer_shared/src/parser/type_info.dart';
 import 'package:_fe_analyzer_shared/src/parser/type_info_impl.dart';
@@ -298,12 +296,12 @@
 
   void test_prefixedTypeInfo() {
     final Token start = scanString('before C.a ;').tokens;
-    final Token expectedEnd = start.next.next.next;
+    final Token expectedEnd = start.next!.next!.next!;
 
     expect(prefixedType.skipType(start), expectedEnd);
     expect(prefixedType.couldBeExpression, isTrue);
 
-    TypeInfoListener listener;
+    late TypeInfoListener listener;
     void assertResult(Token actualEnd) {
       expect(actualEnd, expectedEnd);
       expect(listener.calls, [
@@ -374,12 +372,12 @@
 
   void test_simpleNullableType() {
     final Token start = scanString('before C? ;').tokens;
-    final Token expectedEnd = start.next.next;
+    final Token expectedEnd = start.next!.next!;
 
     expect(simpleNullableType.skipType(start), expectedEnd);
     expect(simpleNullableType.couldBeExpression, isTrue);
 
-    TypeInfoListener listener;
+    late TypeInfoListener listener;
     void assertResult(Token actualEnd) {
       expect(actualEnd, expectedEnd);
       expect(listener.calls, [
@@ -424,13 +422,13 @@
 
   void test_gt_questionMark() {
     final Token start = scanString('before C<T>? ;').tokens;
-    final Token expectedEnd = start.next.next.next.next.next;
+    final Token expectedEnd = start.next!.next!.next!.next!.next!;
     expect(expectedEnd.lexeme, '?');
 
     expect(simpleNullableTypeWith1Argument.skipType(start), expectedEnd);
     expect(simpleNullableTypeWith1Argument.couldBeExpression, isFalse);
 
-    TypeInfoListener listener;
+    late TypeInfoListener listener;
     void assertResult(Token actualEnd) {
       expect(actualEnd, expectedEnd);
       expect(listener.calls, [
@@ -495,12 +493,12 @@
 
   void test_simpleType() {
     final Token start = scanString('before C ;').tokens;
-    final Token expectedEnd = start.next;
+    final Token expectedEnd = start.next!;
 
     expect(simpleType.skipType(start), expectedEnd);
     expect(simpleType.couldBeExpression, isTrue);
 
-    TypeInfoListener listener;
+    late TypeInfoListener listener;
     void assertResult(Token actualEnd) {
       expect(actualEnd, expectedEnd);
       expect(listener.calls, [
@@ -568,13 +566,13 @@
 
   void test_gt() {
     final Token start = scanString('before C<T> ;').tokens;
-    final Token expectedEnd = start.next.next.next.next;
+    final Token expectedEnd = start.next!.next!.next!.next!;
     expect(expectedEnd.lexeme, '>');
 
     expect(simpleTypeWith1Argument.skipType(start), expectedEnd);
     expect(simpleTypeWith1Argument.couldBeExpression, isFalse);
 
-    TypeInfoListener listener;
+    late TypeInfoListener listener;
     void assertResult(Token actualEnd) {
       expect(actualEnd, expectedEnd);
       expect(listener.calls, [
@@ -608,21 +606,21 @@
 
   void test_gt_eq() {
     final Token start = scanString('before C<T>= ;').tokens;
-    final Token t = start.next.next.next;
-    final Token semicolon = t.next.next;
+    final Token t = start.next!.next!.next!;
+    final Token semicolon = t.next!.next!;
     expect(semicolon.lexeme, ';');
 
     Token skip = simpleTypeWith1ArgumentGtEq.skipType(start);
     expect(skip.lexeme, '>');
-    expect(skip.next.lexeme, '=');
-    expect(skip.next.next, semicolon);
+    expect(skip.next!.lexeme, '=');
+    expect(skip.next!.next, semicolon);
     expect(simpleTypeWith1ArgumentGtEq.couldBeExpression, isFalse);
 
-    TypeInfoListener listener;
+    late TypeInfoListener listener;
     void assertResult(Token actualEnd) {
       expect(actualEnd.lexeme, '>');
-      expect(actualEnd.next.lexeme, '=');
-      expect(actualEnd.next.next, semicolon);
+      expect(actualEnd.next!.lexeme, '=');
+      expect(actualEnd.next!.next, semicolon);
       expect(listener.calls, [
         'handleIdentifier C typeReference',
         'beginTypeArguments <',
@@ -654,20 +652,20 @@
 
   void test_gt_gt() {
     final Token start = scanString('before C<T>> ;').tokens;
-    final Token semicolon = start.next.next.next.next.next;
+    final Token semicolon = start.next!.next!.next!.next!.next!;
     expect(semicolon.lexeme, ';');
 
     Token skip = simpleTypeWith1ArgumentGtGt.skipType(start);
     expect(skip.lexeme, '>');
-    expect(skip.next.lexeme, '>');
-    expect(skip.next.next, semicolon);
+    expect(skip.next!.lexeme, '>');
+    expect(skip.next!.next, semicolon);
     expect(simpleTypeWith1ArgumentGtGt.couldBeExpression, isFalse);
 
-    TypeInfoListener listener;
+    late TypeInfoListener listener;
     void assertResult(Token actualEnd) {
       expect(actualEnd.lexeme, '>');
-      expect(actualEnd.next.lexeme, '>');
-      expect(actualEnd.next.next, semicolon);
+      expect(actualEnd.next!.lexeme, '>');
+      expect(actualEnd.next!.next, semicolon);
       expect(listener.calls, [
         'handleIdentifier C typeReference',
         'beginTypeArguments <',
@@ -1592,7 +1590,7 @@
     expect(simpleTypeArgument1.typeInfo, simpleTypeWith1Argument);
 
     final Token start = scanString('before <T> after').tokens;
-    final Token gt = start.next.next.next;
+    final Token gt = start.next!.next!.next!;
     expect(gt.lexeme, '>');
 
     Token skip = simpleTypeArgument1.skip(start);
@@ -1606,14 +1604,14 @@
     expect(simpleTypeArgument1GtEq.typeInfo, simpleTypeWith1ArgumentGtEq);
 
     final Token start = scanString('before <T>= after').tokens;
-    Token t = start.next.next;
-    expect(t.next.lexeme, '>=');
+    Token t = start.next!.next!;
+    expect(t.next!.lexeme, '>=');
 
     Token skip = simpleTypeArgument1GtEq.skip(start);
     validateTokens(start);
     expect(skip.lexeme, '>');
-    expect(skip.next.lexeme, '=');
-    expect(skip.next.next, t.next.next);
+    expect(skip.next!.lexeme, '=');
+    expect(skip.next!.next, t.next!.next);
   }
 
   void test_basic_gt_gt() {
@@ -1621,17 +1619,17 @@
     expect(simpleTypeArgument1GtGt.typeArgumentCount, 1);
     expect(simpleTypeArgument1GtGt.typeInfo, simpleTypeWith1ArgumentGtGt);
 
-    final Token start = scanString('before <S<T>> after').tokens.next.next;
-    var gtgt = start.next.next.next;
+    final Token start = scanString('before <S<T>> after').tokens.next!.next!;
+    var gtgt = start.next!.next!.next!;
     expect(gtgt.lexeme, '>>');
-    Token after = gtgt.next;
+    Token after = gtgt.next!;
     expect(after.lexeme, 'after');
 
     Token skip = simpleTypeArgument1GtGt.skip(start);
     validateTokens(start);
     expect(skip.lexeme, '>');
-    expect(skip.next.lexeme, '>');
-    expect(skip.next.next, after);
+    expect(skip.next!.lexeme, '>');
+    expect(skip.next!.next, after);
   }
 
   void test_compute_gt() {
@@ -1644,9 +1642,9 @@
 
   void test_compute_gt_gt() {
     String source = '<C<T>>';
-    Token start = scan(source).next.next;
+    Token start = scan(source).next!.next!;
     expect(start.lexeme, 'C');
-    Token gtgt = start.next.next.next;
+    Token gtgt = start.next!.next!.next!;
     expect(gtgt.lexeme, '>>');
 
     expect(computeTypeParamOrArg(start, false), simpleTypeArgument1GtGt);
@@ -1654,9 +1652,9 @@
   }
 
   void testParseArguments(TypeParamOrArgInfo typeArg, String source,
-      [String next]) {
+      [String? next]) {
     final Token start = scanString('before $source after').tokens;
-    final Token after = start.next.next.next.next;
+    final Token after = start.next!.next!.next!.next!;
     expect(after.lexeme, 'after');
     final TypeInfoListener listener = new TypeInfoListener();
 
@@ -1666,10 +1664,10 @@
             useImplicitCreationExpression: useImplicitCreationExpressionInCfe));
     validateTokens(start);
     expect(token.lexeme, '>');
-    token = token.next;
+    token = token.next!;
     if (next != null) {
       expect(token.lexeme, next);
-      token = token.next;
+      token = token.next!;
     }
     expect(token, after);
     expect(listener.calls, [
@@ -1695,9 +1693,9 @@
   }
 
   void testParseVariables(TypeParamOrArgInfo typeParam, String source,
-      [String next]) {
+      [String? next]) {
     final Token start = scanString('before $source after').tokens;
-    final Token after = start.next.next.next.next;
+    final Token after = start.next!.next!.next!.next!;
     expect(after.lexeme, 'after');
     final TypeInfoListener listener = new TypeInfoListener();
 
@@ -1707,10 +1705,10 @@
             useImplicitCreationExpression: useImplicitCreationExpressionInCfe));
     validateTokens(start);
     expect(token.lexeme, '>');
-    token = token.next;
+    token = token.next!;
     if (next != null) {
       expect(token.lexeme, next);
-      token = token.next;
+      token = token.next!;
     }
     expect(token, after);
     expect(listener.calls, [
@@ -2588,7 +2586,7 @@
   }
 }
 
-void expectInfo(expectedInfo, String source, {bool required}) {
+void expectInfo(expectedInfo, String source, {bool? required}) {
   if (required == null) {
     compute(expectedInfo, source, scan(source), true);
     compute(expectedInfo, source, scan(source), false);
@@ -2600,12 +2598,12 @@
 /// Note that if [required] is null it is run both with required [true] and
 /// [false] and expect the same in both situations.
 void expectComplexInfo(String source,
-    {bool required,
+    {bool? required,
     bool inDeclaration = false,
     bool couldBeExpression = false,
-    String expectedAfter,
-    List<String> expectedCalls,
-    List<ExpectedError> expectedErrors}) {
+    String? expectedAfter,
+    List<String>? expectedCalls,
+    List<ExpectedError>? expectedErrors}) {
   if (required == null) {
     computeComplex(source, scan(source), true, inDeclaration, couldBeExpression,
         expectedAfter, expectedCalls, expectedErrors);
@@ -2619,7 +2617,7 @@
 
 void expectNestedInfo(expectedInfo, String source) {
   expect(source.startsWith('<'), isTrue);
-  Token start = scan(source).next;
+  Token start = scan(source).next!;
   compute(expectedInfo, source, start, true);
 }
 
@@ -2643,13 +2641,13 @@
     bool required,
     bool inDeclaration,
     bool couldBeExpression,
-    String expectedAfter,
-    List<String> expectedCalls,
-    List<ExpectedError> expectedErrors) {
+    String? expectedAfter,
+    List<String>? expectedCalls,
+    List<ExpectedError>? expectedErrors) {
   int expectedGtGtAndNullEndCount = countGtGtAndNullEnd(start);
   ComplexTypeInfo typeInfo = compute(
       const TypeMatcher<ComplexTypeInfo>(), source, start, required,
-      inDeclaration: inDeclaration);
+      inDeclaration: inDeclaration) as ComplexTypeInfo;
   expect(typeInfo.start, start.next, reason: source);
   expect(typeInfo.couldBeExpression, couldBeExpression);
   expectEnd(expectedAfter, typeInfo.skipType(start));
@@ -2673,16 +2671,16 @@
 void expectComplexTypeArg(String source,
     {bool inDeclaration = false,
     int typeArgumentCount = -1,
-    String expectedAfter,
-    List<String> expectedCalls,
-    List<ExpectedError> expectedErrors}) {
+    String? expectedAfter,
+    List<String>? expectedCalls,
+    List<ExpectedError>? expectedErrors}) {
   Token start = scan(source);
   int expectedGtGtAndNullEndCount = countGtGtAndNullEnd(start);
   ComplexTypeParamOrArgInfo typeVarInfo = computeVar(
       const TypeMatcher<ComplexTypeParamOrArgInfo>(),
       source,
       start,
-      inDeclaration);
+      inDeclaration) as ComplexTypeParamOrArgInfo;
 
   expect(typeVarInfo.start, start.next, reason: source);
   expectEnd(expectedAfter, typeVarInfo.skip(start));
@@ -2708,16 +2706,16 @@
 void expectComplexTypeParam(String source,
     {bool inDeclaration = false,
     int typeArgumentCount = -1,
-    String expectedAfter,
-    List<String> expectedCalls,
-    List<ExpectedError> expectedErrors}) {
+    String? expectedAfter,
+    List<String>? expectedCalls,
+    List<ExpectedError>? expectedErrors}) {
   Token start = scan(source);
   int expectedGtGtAndNullEndCount = countGtGtAndNullEnd(start);
   ComplexTypeParamOrArgInfo typeVarInfo = computeVar(
       const TypeMatcher<ComplexTypeParamOrArgInfo>(),
       source,
       start,
-      inDeclaration);
+      inDeclaration) as ComplexTypeParamOrArgInfo;
 
   expect(typeVarInfo.start, start.next, reason: source);
   expectEnd(expectedAfter, typeVarInfo.skip(start));
@@ -2743,9 +2741,9 @@
 
 void expectTypeParamOrArg(expectedInfo, String source,
     {bool inDeclaration = false,
-    String expectedAfter,
-    List<String> expectedCalls,
-    List<ExpectedError> expectedErrors}) {
+    String? expectedAfter,
+    List<String>? expectedCalls,
+    List<ExpectedError>? expectedErrors}) {
   Token start = scan(source);
   computeVar(expectedInfo, source, start, inDeclaration);
 }
@@ -2761,21 +2759,21 @@
   return typeVarInfo;
 }
 
-void expectEnd(String tokenAfter, Token end) {
+void expectEnd(String? tokenAfter, Token end) {
   if (tokenAfter == null) {
     expect(end.isEof, isFalse);
-    if (!end.next.isEof) {
+    if (!end.next!.isEof) {
       fail('Expected EOF after $end but found ${end.next}');
     }
   } else {
-    expect(end.next.lexeme, tokenAfter);
+    expect(end.next!.lexeme, tokenAfter);
   }
 }
 
 Token scan(String source) {
   Token start = scanString(source).tokens;
   while (start is ErrorToken) {
-    start = start.next;
+    start = start.next!;
   }
   return new SyntheticToken(TokenType.EOF, -1)..setNext(start);
 }
@@ -2787,18 +2785,18 @@
         optional('>>', token)) {
       ++count;
     }
-    token = token.next;
+    token = token.next!;
   }
   return count;
 }
 
 void validateTokens(Token token) {
   int count = 0;
-  if (token.isEof && !token.next.isEof) {
-    token = token.next;
+  if (token.isEof && !token.next!.isEof) {
+    token = token.next!;
   }
   while (!token.isEof) {
-    Token next = token.next;
+    Token next = token.next!;
     expect(token.charOffset, lessThanOrEqualTo(next.charOffset));
     expect(next.previous, token, reason: next.type.toString());
     if (next is SyntheticToken) {
@@ -2813,12 +2811,12 @@
 class TypeInfoListener implements Listener {
   final bool metadataAllowed;
   List<String> calls = <String>[];
-  List<ExpectedError> errors;
-  Token firstToken;
+  List<ExpectedError>? errors;
+  Token? firstToken;
 
   TypeInfoListener({this.firstToken, this.metadataAllowed: false}) {
-    if (firstToken != null && firstToken.isEof) {
-      firstToken = firstToken.next;
+    if (firstToken != null && firstToken!.isEof) {
+      firstToken = firstToken!.next;
     }
   }
 
@@ -2832,8 +2830,8 @@
   }
 
   @override
-  void beginFormalParameter(Token token, MemberKind kind, Token requiredToken,
-      Token covariantToken, Token varFinalOrConst) {
+  void beginFormalParameter(Token token, MemberKind kind, Token? requiredToken,
+      Token? covariantToken, Token? varFinalOrConst) {
     // TODO(danrubel): Update tests to include required and covariant
     calls.add('beginFormalParameter $token $kind');
   }
@@ -2894,12 +2892,12 @@
 
   @override
   void endFormalParameter(
-      Token thisKeyword,
-      Token superKeyword,
-      Token periodAfterThisOrSuper,
+      Token? thisKeyword,
+      Token? superKeyword,
+      Token? periodAfterThisOrSuper,
       Token nameToken,
-      Token initializerStart,
-      Token initializerEnd,
+      Token? initializerStart,
+      Token? initializerEnd,
       FormalParameterKind kind,
       MemberKind memberKind) {
     calls.add('endFormalParameter $thisKeyword $periodAfterThisOrSuper '
@@ -2907,12 +2905,12 @@
   }
 
   @override
-  void endFunctionType(Token functionToken, Token questionMark) {
+  void endFunctionType(Token functionToken, Token? questionMark) {
     calls.add('endFunctionType $functionToken $questionMark');
   }
 
   @override
-  void endMetadata(Token beginToken, Token periodBeforeName, Token endToken) {
+  void endMetadata(Token beginToken, Token? periodBeforeName, Token endToken) {
     if (metadataAllowed) {
       calls.add('endMetadata $beginToken $periodBeforeName $endToken');
     } else {
@@ -2934,7 +2932,7 @@
 
   @override
   void endTypeVariable(
-      Token token, int index, Token extendsOrSuper, Token variance) {
+      Token token, int index, Token? extendsOrSuper, Token? variance) {
     calls.add('endTypeVariable $token $index $extendsOrSuper $variance');
     assertTokenInStream(token);
     assertTokenInStream(extendsOrSuper);
@@ -2989,9 +2987,9 @@
   @override
   void handleRecoverableError(
       Message message, Token startToken, Token endToken) {
-    errors ??= <ExpectedError>[];
     int offset = startToken.charOffset;
-    errors.add(error(message.code, offset, endToken.charEnd - offset));
+    (errors ??= <ExpectedError>[])
+        .add(error(message.code, offset, endToken.charEnd - offset));
   }
 
   @override
@@ -3000,7 +2998,7 @@
   }
 
   @override
-  void handleType(Token beginToken, Token questionMark) {
+  void handleType(Token beginToken, Token? questionMark) {
     calls.add('handleType $beginToken $questionMark');
   }
 
@@ -3024,20 +3022,20 @@
     throw '${invocation.memberName} should not be called.';
   }
 
-  void assertTokenInStream(Token match) {
+  void assertTokenInStream(Token? match) {
     if (firstToken != null && match != null && !match.isEof) {
-      Token token = firstToken;
+      Token token = firstToken!;
       while (!token.isEof) {
         if (identical(token, match)) {
           return;
         }
-        token = token.next;
+        token = token.next!;
       }
       final msg = new StringBuffer();
       msg.writeln('Expected $match in token stream, but found');
       while (!token.isEof) {
         msg.write(' $token');
-        token = token.next;
+        token = token.next!;
       }
       fail(msg.toString());
     }
diff --git a/pkg/front_end/test/fasta/scanner/scanner_suite.dart b/pkg/front_end/test/fasta/scanner/scanner_suite.dart
index f9822e8..69ea079 100644
--- a/pkg/front_end/test/fasta/scanner/scanner_suite.dart
+++ b/pkg/front_end/test/fasta/scanner/scanner_suite.dart
@@ -2,8 +2,6 @@
 // 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.
 
-// @dart = 2.9
-
 import 'package:testing/testing.dart' show Chain, ChainContext, Step, runMe;
 
 import '../../utils/scanner_chain.dart' show Read, Scan;
diff --git a/pkg/front_end/test/fasta/type_inference/factor_type_test.dart b/pkg/front_end/test/fasta/type_inference/factor_type_test.dart
index a365e93..a743918 100644
--- a/pkg/front_end/test/fasta/type_inference/factor_type_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/factor_type_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:_fe_analyzer_shared/src/flow_analysis/factory_type_test_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:front_end/src/api_prototype/compiler_options.dart';
@@ -106,8 +104,10 @@
   CompilerOptions options = new CompilerOptions()
     ..explicitExperimentalFlags[ExperimentalFlag.nonNullable] = true;
   InternalCompilerResult result = await compileScript('',
-      options: options, requireMain: false, retainDataForTesting: true);
+      options: options,
+      requireMain: false,
+      retainDataForTesting: true) as InternalCompilerResult;
   new FactorTypeTest(
-          new TypeEnvironment(result.coreTypes, result.classHierarchy))
+          new TypeEnvironment(result.coreTypes!, result.classHierarchy!))
       .run();
 }
diff --git a/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_nnbd_test.dart b/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_nnbd_test.dart
index 7bf3df7..f037ae5 100644
--- a/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_nnbd_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_nnbd_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_constraint_gatherer.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema_environment.dart';
@@ -22,15 +20,15 @@
 
 @reflectiveTest
 class TypeConstraintGathererTest {
-  Env env;
+  late Env env;
 
   final Map<String, DartType Function()> additionalTypes = {
     'UNKNOWN': () => new UnknownType()
   };
 
-  Library _coreLibrary;
+  late Library _coreLibrary;
 
-  Library _testLibrary;
+  late Library _testLibrary;
 
   TypeConstraintGathererTest();
 
@@ -308,7 +306,7 @@
   }
 
   void checkConstraintsLower(String type, String bound, List<String> expected,
-      {String typeParameters, String typeParametersToConstrain}) {
+      {String? typeParameters, String? typeParametersToConstrain}) {
     env.withTypeParameters(typeParameters ?? '',
         (List<TypeParameter> typeParameterNodes) {
       List<TypeParameter> typeParameterNodesToConstrain;
@@ -345,8 +343,8 @@
         typeParameterNodesToConstrain);
   }
 
-  void checkConstraintsUpper(String type, String bound, List<String> expected,
-      {String typeParameters, String typeParametersToConstrain}) {
+  void checkConstraintsUpper(String type, String bound, List<String>? expected,
+      {String? typeParameters, String? typeParametersToConstrain}) {
     env.withTypeParameters(typeParameters ?? '',
         (List<TypeParameter> typeParameterNodes) {
       List<TypeParameter> typeParameterNodesToConstrain;
@@ -372,7 +370,7 @@
       DartType type,
       DartType bound,
       Library clientLibrary,
-      List<String> expectedConstraints,
+      List<String>? expectedConstraints,
       List<TypeParameter> typeParameterNodesToConstrain) {
     _checkConstraintsHelper(
         type,
@@ -387,7 +385,7 @@
       DartType a,
       DartType b,
       Library clientLibrary,
-      List<String> expectedConstraints,
+      List<String>? expectedConstraints,
       bool Function(TypeConstraintGatherer, DartType, DartType) tryConstrain,
       List<TypeParameter> typeParameterNodesToConstrain) {
     var typeSchemaEnvironment = new TypeSchemaEnvironment(
@@ -403,7 +401,7 @@
     }
     expect(constraints, isNotNull);
     var constraintStrings = <String>[];
-    constraints.forEach((t, constraint) {
+    constraints!.forEach((t, constraint) {
       if (constraint.lower is! UnknownType ||
           constraint.upper is! UnknownType) {
         var s = t.name;
@@ -413,7 +411,7 @@
         if (constraint.upper is! UnknownType) {
           s = '$s <: ${typeSchemaToString(constraint.upper)}';
         }
-        constraintStrings.add(s);
+        constraintStrings.add(s as String);
       }
     });
     expect(constraintStrings, unorderedEquals(expectedConstraints));
diff --git a/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_test.dart b/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_test.dart
index 16df20d..e42fc10 100644
--- a/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_constraint_gatherer_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_constraint_gatherer.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema_environment.dart';
@@ -22,15 +20,15 @@
 
 @reflectiveTest
 class TypeConstraintGathererTest {
-  Env env;
+  late Env env;
 
   final Map<String, DartType Function()> additionalTypes = {
     'UNKNOWN': () => UnknownType()
   };
 
-  Library _coreLibrary;
+  late Library _coreLibrary;
 
-  Library _testLibrary;
+  late Library _testLibrary;
 
   TypeConstraintGathererTest();
 
@@ -215,8 +213,8 @@
         typeParameters: 'T1 extends Object*');
   }
 
-  void checkConstraintsLower(String type, String bound, List<String> expected,
-      {String typeParameters, String typeParametersToConstrain}) {
+  void checkConstraintsLower(String type, String bound, List<String>? expected,
+      {String? typeParameters, String? typeParametersToConstrain}) {
     env.withTypeParameters(typeParameters ?? '',
         (List<TypeParameter> typeParameterNodes) {
       List<TypeParameter> typeParameterNodesToConstrain;
@@ -242,7 +240,7 @@
       DartType type,
       DartType bound,
       Library clientLibrary,
-      List<String> expectedConstraints,
+      List<String>? expectedConstraints,
       List<TypeParameter> typeParameterNodesToConstrain) {
     _checkConstraintsHelper(
         type,
@@ -253,8 +251,8 @@
         typeParameterNodesToConstrain);
   }
 
-  void checkConstraintsUpper(String type, String bound, List<String> expected,
-      {String typeParameters, String typeParametersToConstrain}) {
+  void checkConstraintsUpper(String type, String bound, List<String>? expected,
+      {String? typeParameters, String? typeParametersToConstrain}) {
     env.withTypeParameters(typeParameters ?? '',
         (List<TypeParameter> typeParameterNodes) {
       List<TypeParameter> typeParameterNodesToConstrain;
@@ -280,7 +278,7 @@
       DartType type,
       DartType bound,
       Library clientLibrary,
-      List<String> expectedConstraints,
+      List<String>? expectedConstraints,
       List<TypeParameter> typeParameterNodesToConstrain) {
     _checkConstraintsHelper(
         type,
@@ -295,7 +293,7 @@
       DartType a,
       DartType b,
       Library clientLibrary,
-      List<String> expectedConstraints,
+      List<String>? expectedConstraints,
       bool Function(TypeConstraintGatherer, DartType, DartType) tryConstrain,
       List<TypeParameter> typeParameterNodesToConstrain) {
     var typeSchemaEnvironment = new TypeSchemaEnvironment(
@@ -311,7 +309,7 @@
     }
     expect(constraints, isNotNull);
     var constraintStrings = <String>[];
-    constraints.forEach((t, constraint) {
+    constraints!.forEach((t, constraint) {
       if (constraint.lower is! UnknownType ||
           constraint.upper is! UnknownType) {
         var s = t.name;
@@ -321,7 +319,7 @@
         if (constraint.upper is! UnknownType) {
           s = '$s <: ${typeSchemaToString(constraint.upper)}';
         }
-        constraintStrings.add(s);
+        constraintStrings.add(s as String);
       }
     });
     expect(constraintStrings, unorderedEquals(expectedConstraints));
diff --git a/pkg/front_end/test/fasta/type_inference/type_inference_engine_test.dart b/pkg/front_end/test/fasta/type_inference/type_inference_engine_test.dart
index 0588e94..0221278 100644
--- a/pkg/front_end/test/fasta/type_inference/type_inference_engine_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_inference_engine_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_inference_engine.dart';
 import 'package:kernel/ast.dart';
 import 'package:test/test.dart';
@@ -226,7 +224,7 @@
         isFalse);
   }
 
-  TypeParameterType tpt(TypeParameter param, {int variance = null}) {
+  TypeParameterType tpt(TypeParameter param, {int? variance = null}) {
     return new TypeParameterType(param, Nullability.legacy)
       ..parameter.variance = variance;
   }
diff --git a/pkg/front_end/test/fasta/type_inference/type_schema_elimination_nnbd_test.dart b/pkg/front_end/test/fasta/type_inference/type_schema_elimination_nnbd_test.dart
index dc5b7cb..7abe586 100644
--- a/pkg/front_end/test/fasta/type_inference/type_schema_elimination_nnbd_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_schema_elimination_nnbd_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema_elimination.dart'
     as typeSchemaElimination;
diff --git a/pkg/front_end/test/fasta/type_inference/type_schema_elimination_test.dart b/pkg/front_end/test/fasta/type_inference/type_schema_elimination_test.dart
index 36f0c1a..49f3b77 100644
--- a/pkg/front_end/test/fasta/type_inference/type_schema_elimination_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_schema_elimination_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema_elimination.dart'
     as typeSchemaElimination;
diff --git a/pkg/front_end/test/fasta/type_inference/type_schema_environment_nnbd_test.dart b/pkg/front_end/test/fasta/type_inference/type_schema_environment_nnbd_test.dart
index 20d575a..182d44b 100644
--- a/pkg/front_end/test/fasta/type_inference/type_schema_environment_nnbd_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_schema_environment_nnbd_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema_environment.dart';
 import 'package:kernel/ast.dart';
@@ -21,15 +19,15 @@
 
 @reflectiveTest
 class TypeSchemaEnvironmentTest {
-  Env typeParserEnvironment;
-  TypeSchemaEnvironment typeSchemaEnvironment;
+  late Env typeParserEnvironment;
+  late TypeSchemaEnvironment typeSchemaEnvironment;
 
   final Map<String, DartType Function()> additionalTypes = {
     "UNKNOWN": () => new UnknownType(),
   };
 
-  Library _coreLibrary;
-  Library _testLibrary;
+  late Library _coreLibrary;
+  late Library _testLibrary;
 
   Library get coreLibrary => _coreLibrary;
   Library get testLibrary => _testLibrary;
@@ -111,7 +109,7 @@
   /// There's an infinite amount of such types, and the list contains some
   /// practical base cases.  For the definition of TOP see the following:
   /// https://github.com/dart-lang/language/blob/master/resources/type-system/upper-lower-bounds.md#helper-predicates
-  static const Map<String, String> topPredicateEnumeration = <String, String>{
+  static const Map<String, String?> topPredicateEnumeration = {
     // dynamic and void.
     "dynamic": null,
     "void": null,
@@ -210,7 +208,7 @@
   /// There's an infinite amount of such types, and the list contains some
   /// practical base cases.  For the definition of OBJECT see the following:
   /// https://github.com/dart-lang/language/blob/master/resources/type-system/upper-lower-bounds.md#helper-predicates
-  static const Map<String, String> objectPredicateEnumeration = {
+  static const Map<String, String?> objectPredicateEnumeration = {
     "Object": null,
     "FutureOr<Object>": null,
     "FutureOr<FutureOr<Object>>": null,
@@ -224,7 +222,7 @@
   ///
   /// The names of the variables here and in [nullPredicateEnumeration] should
   /// be distinct to avoid collisions.
-  static const Map<String, String> bottomPredicateEnumeration = {
+  static const Map<String, String?> bottomPredicateEnumeration = {
     "Never": null,
     "Xb & Never": "Xb extends Object?",
     "Yb & Zb & Never": "Yb extends Object?, Zb extends Object?",
@@ -241,7 +239,7 @@
   ///
   /// The names of the variables here and in [bottomPredicateEnumeration] should
   /// be distinct to avoid collisions.
-  static const Map<String, String> nullPredicateEnumeration = {
+  static const Map<String, String?> nullPredicateEnumeration = {
     // T? where BOTTOM(T).
     "Never?": null,
     "Xn?": "Xn extends Never",
@@ -256,8 +254,8 @@
     "Null": null,
   };
 
-  static String joinTypeParameters(
-      String typeParameters1, String typeParameters2) {
+  static String? joinTypeParameters(
+      String? typeParameters1, String? typeParameters2) {
     if (typeParameters1 == null) return typeParameters2;
     if (typeParameters2 == null) return typeParameters1;
     if (typeParameters1 == typeParameters2) return typeParameters1;
@@ -277,7 +275,7 @@
     //   T2 otherwise
     for (String t1 in bottomPredicateEnumeration.keys) {
       for (String t2 in bottomPredicateEnumeration.keys) {
-        String typeParameters = joinTypeParameters(
+        String? typeParameters = joinTypeParameters(
             bottomPredicateEnumeration[t1], bottomPredicateEnumeration[t2]);
         typeParserEnvironment.withTypeParameters(typeParameters, (_) {
           String expected =
@@ -320,7 +318,7 @@
     //   T2 otherwise
     for (String t1 in nullPredicateEnumeration.keys) {
       for (String t2 in nullPredicateEnumeration.keys) {
-        String typeParameters = joinTypeParameters(
+        String? typeParameters = joinTypeParameters(
             nullPredicateEnumeration[t1], nullPredicateEnumeration[t2]);
         typeParserEnvironment.withTypeParameters(typeParameters, (_) {
           String expected =
@@ -600,7 +598,7 @@
     //   T2 otherwise
     for (String t1 in topPredicateEnumeration.keys) {
       for (String t2 in topPredicateEnumeration.keys) {
-        String typeParameters = joinTypeParameters(
+        String? typeParameters = joinTypeParameters(
             topPredicateEnumeration[t1], topPredicateEnumeration[t2]);
         typeParserEnvironment.withTypeParameters(typeParameters, (_) {
           String expected =
@@ -1100,7 +1098,7 @@
     //   T2 otherwise
     for (String t1 in topPredicateEnumeration.keys) {
       for (String t2 in topPredicateEnumeration.keys) {
-        String typeParameters = joinTypeParameters(
+        String? typeParameters = joinTypeParameters(
             topPredicateEnumeration[t1], topPredicateEnumeration[t2]);
         typeParserEnvironment.withTypeParameters(typeParameters, (_) {
           String expected =
@@ -1143,7 +1141,7 @@
     //   T2 otherwise
     for (String t1 in objectPredicateEnumeration.keys) {
       for (String t2 in objectPredicateEnumeration.keys) {
-        String typeParameters = joinTypeParameters(
+        String? typeParameters = joinTypeParameters(
             objectPredicateEnumeration[t1], objectPredicateEnumeration[t2]);
         typeParserEnvironment.withTypeParameters(typeParameters, (_) {
           String expected =
@@ -1194,7 +1192,7 @@
     //   T1 otherwise
     for (String t1 in bottomPredicateEnumeration.keys) {
       for (String t2 in bottomPredicateEnumeration.keys) {
-        String typeParameters = joinTypeParameters(
+        String? typeParameters = joinTypeParameters(
             bottomPredicateEnumeration[t1], bottomPredicateEnumeration[t2]);
         typeParserEnvironment.withTypeParameters(typeParameters, (_) {
           String expected =
@@ -1237,7 +1235,7 @@
     //   T1 otherwise
     for (String t1 in nullPredicateEnumeration.keys) {
       for (String t2 in nullPredicateEnumeration.keys) {
-        String typeParameters = joinTypeParameters(
+        String? typeParameters = joinTypeParameters(
             nullPredicateEnumeration[t1], nullPredicateEnumeration[t2]);
         typeParserEnvironment.withTypeParameters(typeParameters, (_) {
           String expected =
@@ -1505,8 +1503,7 @@
   }
 
   void checkConstraintSolving(String constraint, String expected,
-      {bool grounded}) {
-    assert(grounded != null);
+      {required bool grounded}) {
     expect(
         typeSchemaEnvironment.solveTypeConstraint(
             parseConstraint(constraint),
@@ -1516,17 +1513,13 @@
         parseType(expected));
   }
 
-  void checkConstraintUpperBound({String constraint, String bound}) {
-    assert(constraint != null);
-    assert(bound != null);
-
+  void checkConstraintUpperBound(
+      {required String constraint, required String bound}) {
     expect(parseConstraint(constraint).upper, parseType(bound));
   }
 
-  void checkConstraintLowerBound({String constraint, String bound}) {
-    assert(constraint != null);
-    assert(bound != null);
-
+  void checkConstraintLowerBound(
+      {required String constraint, required String bound}) {
     expect(parseConstraint(constraint).lower, parseType(bound));
   }
 
@@ -1572,16 +1565,12 @@
   }
 
   void checkUpperBound(
-      {String type1,
-      String type2,
-      String upperBound,
-      String typeParameters,
+      {required String type1,
+      required String type2,
+      required String upperBound,
+      String? typeParameters,
       bool nonNull1: false,
       bool nonNull2: false}) {
-    assert(type1 != null);
-    assert(type2 != null);
-    assert(upperBound != null);
-
     typeParserEnvironment.withTypeParameters(typeParameters,
         (List<TypeParameter> typeParameterNodes) {
       DartType dartType1 = parseType(type1);
@@ -1600,11 +1589,10 @@
   }
 
   void checkLowerBound(
-      {String type1, String type2, String lowerBound, String typeParameters}) {
-    assert(type1 != null);
-    assert(type2 != null);
-    assert(lowerBound != null);
-
+      {required String type1,
+      required String type2,
+      required String lowerBound,
+      String? typeParameters}) {
     typeParserEnvironment.withTypeParameters(typeParameters,
         (List<TypeParameter> typeParameterNodes) {
       expect(
@@ -1615,28 +1603,23 @@
   }
 
   void checkInference(
-      {String typeParametersToInfer,
-      String functionType,
-      String actualParameterTypes,
-      String returnContextType,
-      String inferredTypesFromDownwardPhase,
-      String expectedTypes}) {
-    assert(typeParametersToInfer != null);
-    assert(functionType != null);
-    assert(expectedTypes != null);
-
+      {required String typeParametersToInfer,
+      required String functionType,
+      String? actualParameterTypes,
+      String? returnContextType,
+      String? inferredTypesFromDownwardPhase,
+      required String expectedTypes}) {
     typeParserEnvironment.withTypeParameters(typeParametersToInfer,
         (List<TypeParameter> typeParameterNodesToInfer) {
-      FunctionType functionTypeNode = parseType(functionType);
-      DartType returnContextTypeNode =
+      FunctionType functionTypeNode = parseType(functionType) as FunctionType;
+      DartType? returnContextTypeNode =
           returnContextType == null ? null : parseType(returnContextType);
-      List<DartType> actualTypeNodes = actualParameterTypes == null
+      List<DartType>? actualTypeNodes = actualParameterTypes == null
           ? null
           : parseTypes(actualParameterTypes);
-      List<DartType> expectedTypeNodes =
-          expectedTypes == null ? null : parseTypes(expectedTypes);
+      List<DartType> expectedTypeNodes = parseTypes(expectedTypes);
       DartType declaredReturnTypeNode = functionTypeNode.returnType;
-      List<DartType> formalTypeNodes = actualParameterTypes == null
+      List<DartType>? formalTypeNodes = actualParameterTypes == null
           ? null
           : functionTypeNode.positionalParameters;
 
@@ -1668,14 +1651,11 @@
   }
 
   void checkInferenceFromConstraints(
-      {String typeParameter,
-      String constraints,
-      String inferredTypeFromDownwardPhase,
-      bool downwardsInferPhase,
-      String expected}) {
-    assert(typeParameter != null);
-    assert(expected != null);
-    assert(downwardsInferPhase != null);
+      {required String typeParameter,
+      required String constraints,
+      String? inferredTypeFromDownwardPhase,
+      required bool downwardsInferPhase,
+      required String expected}) {
     assert(inferredTypeFromDownwardPhase == null || !downwardsInferPhase);
 
     typeParserEnvironment.withTypeParameters(typeParameter,
diff --git a/pkg/front_end/test/fasta/type_inference/type_schema_environment_test.dart b/pkg/front_end/test/fasta/type_inference/type_schema_environment_test.dart
index 6945778..526e061 100644
--- a/pkg/front_end/test/fasta/type_inference/type_schema_environment_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_schema_environment_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:front_end/src/fasta/type_inference/type_schema_environment.dart';
 import 'package:kernel/ast.dart';
@@ -21,15 +19,15 @@
 
 @reflectiveTest
 class TypeSchemaEnvironmentTest {
-  Env typeParserEnvironment;
-  TypeSchemaEnvironment typeSchemaEnvironment;
+  late Env typeParserEnvironment;
+  late TypeSchemaEnvironment typeSchemaEnvironment;
 
   final Map<String, DartType Function()> additionalTypes = {
     "UNKNOWN": () => new UnknownType(),
   };
 
-  Library _coreLibrary;
-  Library _testLibrary;
+  late Library _coreLibrary;
+  late Library _testLibrary;
 
   Library get coreLibrary => _coreLibrary;
   Library get testLibrary => _testLibrary;
@@ -587,8 +585,7 @@
   }
 
   void checkConstraintSolving(String constraint, String expected,
-      {bool grounded}) {
-    assert(grounded != null);
+      {required bool grounded}) {
     expect(
         typeSchemaEnvironment.solveTypeConstraint(
             parseConstraint(constraint), new DynamicType(), new NullType(),
@@ -596,17 +593,13 @@
         parseType(expected));
   }
 
-  void checkConstraintUpperBound({String constraint, String bound}) {
-    assert(constraint != null);
-    assert(bound != null);
-
+  void checkConstraintUpperBound(
+      {required String constraint, required String bound}) {
     expect(parseConstraint(constraint).upper, parseType(bound));
   }
 
-  void checkConstraintLowerBound({String constraint, String bound}) {
-    assert(constraint != null);
-    assert(bound != null);
-
+  void checkConstraintLowerBound(
+      {required String constraint, required String bound}) {
     expect(parseConstraint(constraint).lower, parseType(bound));
   }
 
@@ -652,11 +645,10 @@
   }
 
   void checkUpperBound(
-      {String type1, String type2, String upperBound, String typeParameters}) {
-    assert(type1 != null);
-    assert(type2 != null);
-    assert(upperBound != null);
-
+      {required String type1,
+      required String type2,
+      required String upperBound,
+      String? typeParameters}) {
     typeParserEnvironment.withTypeParameters(typeParameters,
         (List<TypeParameter> typeParameterNodes) {
       expect(
@@ -667,11 +659,10 @@
   }
 
   void checkLowerBound(
-      {String type1, String type2, String lowerBound, String typeParameters}) {
-    assert(type1 != null);
-    assert(type2 != null);
-    assert(lowerBound != null);
-
+      {required String type1,
+      required String type2,
+      required String lowerBound,
+      String? typeParameters}) {
     typeParserEnvironment.withTypeParameters(typeParameters,
         (List<TypeParameter> typeParameterNodes) {
       expect(
@@ -682,28 +673,23 @@
   }
 
   void checkInference(
-      {String typeParametersToInfer,
-      String functionType,
-      String actualParameterTypes,
-      String returnContextType,
-      String inferredTypesFromDownwardPhase,
-      String expectedTypes}) {
-    assert(typeParametersToInfer != null);
-    assert(functionType != null);
-    assert(expectedTypes != null);
-
+      {required String typeParametersToInfer,
+      required String functionType,
+      String? actualParameterTypes,
+      String? returnContextType,
+      String? inferredTypesFromDownwardPhase,
+      required String expectedTypes}) {
     typeParserEnvironment.withTypeParameters(typeParametersToInfer,
         (List<TypeParameter> typeParameterNodesToInfer) {
-      FunctionType functionTypeNode = parseType(functionType);
-      DartType returnContextTypeNode =
+      FunctionType functionTypeNode = parseType(functionType) as FunctionType;
+      DartType? returnContextTypeNode =
           returnContextType == null ? null : parseType(returnContextType);
-      List<DartType> actualTypeNodes = actualParameterTypes == null
+      List<DartType>? actualTypeNodes = actualParameterTypes == null
           ? null
           : parseTypes(actualParameterTypes);
-      List<DartType> expectedTypeNodes =
-          expectedTypes == null ? null : parseTypes(expectedTypes);
+      List<DartType> expectedTypeNodes = parseTypes(expectedTypes);
       DartType declaredReturnTypeNode = functionTypeNode.returnType;
-      List<DartType> formalTypeNodes = actualParameterTypes == null
+      List<DartType>? formalTypeNodes = actualParameterTypes == null
           ? null
           : functionTypeNode.positionalParameters;
 
@@ -735,14 +721,11 @@
   }
 
   void checkInferenceFromConstraints(
-      {String typeParameter,
-      String constraints,
-      String inferredTypeFromDownwardPhase,
-      bool downwardsInferPhase,
-      String expected}) {
-    assert(typeParameter != null);
-    assert(expected != null);
-    assert(downwardsInferPhase != null);
+      {required String typeParameter,
+      required String constraints,
+      String? inferredTypeFromDownwardPhase,
+      required bool downwardsInferPhase,
+      required String expected}) {
     assert(inferredTypeFromDownwardPhase == null || !downwardsInferPhase);
 
     typeParserEnvironment.withTypeParameters(typeParameter,
diff --git a/pkg/front_end/test/fasta/type_inference/type_schema_test.dart b/pkg/front_end/test/fasta/type_inference/type_schema_test.dart
index eff2502..eb34789 100644
--- a/pkg/front_end/test/fasta/type_inference/type_schema_test.dart
+++ b/pkg/front_end/test/fasta/type_inference/type_schema_test.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.9
-
 import 'package:front_end/src/fasta/type_inference/type_schema.dart';
 import 'package:kernel/ast.dart';
 import 'package:test/test.dart';
@@ -112,38 +110,38 @@
   }
 }
 
-class _OrdinaryVisitor<R> extends Visitor<R> with VisitorNullMixin<R> {
-  final _UnaryFunction<DartType, R> _defaultDartType;
+class _OrdinaryVisitor<R> extends Visitor<R?> with VisitorNullMixin<R> {
+  final _UnaryFunction<DartType, R>? _defaultDartType;
 
-  _OrdinaryVisitor({_UnaryFunction<DartType, R> defaultDartType})
+  _OrdinaryVisitor({_UnaryFunction<DartType, R>? defaultDartType})
       : _defaultDartType = defaultDartType;
 
   @override
-  R defaultDartType(DartType node) {
+  R? defaultDartType(DartType node) {
     if (_defaultDartType != null) {
-      return _defaultDartType(node);
+      return _defaultDartType!(node);
     } else {
       return super.defaultDartType(node);
     }
   }
 }
 
-class _TypeSchemaVisitor<R> extends Visitor<R> with VisitorNullMixin<R> {
-  final _UnaryFunction<DartType, R> _defaultDartType;
-  final _UnaryFunction<UnknownType, R> _visitUnknownType;
+class _TypeSchemaVisitor<R> extends Visitor<R?> with VisitorNullMixin<R> {
+  final _UnaryFunction<DartType, R>? _defaultDartType;
+  final _UnaryFunction<UnknownType, R>? _visitUnknownType;
 
   _TypeSchemaVisitor(
-      {_UnaryFunction<DartType, R> defaultDartType,
-      _UnaryFunction<UnknownType, R> visitUnknownType})
+      {_UnaryFunction<DartType, R>? defaultDartType,
+      _UnaryFunction<UnknownType, R>? visitUnknownType})
       : _defaultDartType = defaultDartType,
         _visitUnknownType = visitUnknownType;
 
   @override
-  R defaultDartType(DartType node) {
+  R? defaultDartType(DartType node) {
     if (node is UnknownType && _visitUnknownType != null) {
-      return _visitUnknownType(node);
+      return _visitUnknownType!(node);
     } else if (_defaultDartType != null) {
-      return _defaultDartType(node);
+      return _defaultDartType!(node);
     } else {
       return super.defaultDartType(node);
     }
diff --git a/tools/VERSION b/tools/VERSION
index 594094a..cd2b3ae 100644
--- a/tools/VERSION
+++ b/tools/VERSION
@@ -27,5 +27,5 @@
 MAJOR 2
 MINOR 16
 PATCH 0
-PRERELEASE 44
+PRERELEASE 45
 PRERELEASE_PATCH 0
\ No newline at end of file