Add warnings when async and async* functions declare a subtype of Future or Stream respectively (is…

pkg/analyzer/lib/src/generated/error_verifier.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 3437 matching lines @@
    * respectively.  If not, report the error using [returnType].
    */
   void _checkForIllegalReturnType(TypeName returnType) {
     if (returnType == null) {
       // No declared return type, so the return type must be dynamic, which is
       // assignable to everything.
       return;
     }
     if (_enclosingFunction.isAsynchronous) {
       if (_enclosingFunction.isGenerator) {
-        if (!_typeSystem.isAssignableTo(
+        if (_options.strongMode) {
+          if (_enclosingFunction.returnType.element !=
+              _typeProvider.streamType.element) {
+            _errorReporter.reportErrorForNode(
+                StaticTypeWarningCode.ILLEGAL_ASYNC_GENERATOR_RETURN_TYPE,
+                returnType);
+          }
+        } else if (!_typeSystem.isAssignableTo(
             _enclosingFunction.returnType, _typeProvider.streamDynamicType)) {
           _errorReporter.reportErrorForNode(
               StaticTypeWarningCode.ILLEGAL_ASYNC_GENERATOR_RETURN_TYPE,
               returnType);
         }
       } else {
-        if (!_typeSystem.isAssignableTo(
+        if (_options.strongMode) {
+          if (_enclosingFunction.returnType.element !=
+              _typeProvider.futureType.element) {
+            _errorReporter.reportErrorForNode(
+                StaticTypeWarningCode.ILLEGAL_ASYNC_RETURN_TYPE, returnType);
+          }
+        } else if (!_typeSystem.isAssignableTo(
             _enclosingFunction.returnType, _typeProvider.futureDynamicType)) {
           _errorReporter.reportErrorForNode(
               StaticTypeWarningCode.ILLEGAL_ASYNC_RETURN_TYPE, returnType);
         }
       }
     } else if (_enclosingFunction.isGenerator) {
       if (!_typeSystem.isAssignableTo(
           _enclosingFunction.returnType, _typeProvider.iterableDynamicType)) {
         _errorReporter.reportErrorForNode(
             StaticTypeWarningCode.ILLEGAL_SYNC_GENERATOR_RETURN_TYPE,
@@ skipping 2686 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }