Fix analyzer type checking of "await" expressions.

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 engine.resolver.error_verifier;
 
 import "dart:math" as math;
 import 'dart:collection';
 
+import 'package:analyzer/src/generated/static_type_analyzer.dart';
+
 import 'ast.dart';
 import 'constant.dart';
 import 'element.dart';
 import 'element_resolver.dart';
 import 'error.dart';
 import 'java_engine.dart';
 import 'parser.dart' show Parser, ParserErrorCode;
 import 'resolver.dart';
 import 'scanner.dart' as sc;
 import 'sdk.dart' show DartSdk, SdkLibrary;
@@ skipping 5858 matching lines @@
     // never allowed to contain return statements with expressions.
     assert(!_inGenerator);
     if (returnExpression == null) {
       if (_enclosingFunction.isAsynchronous) {
         return _typeProvider.futureNullType;
       } else {
         return VoidTypeImpl.instance;
       }
     }
     DartType staticReturnType = getStaticType(returnExpression);
-    if (staticReturnType != null &&
-        _enclosingFunction.isAsynchronous &&
-        staticReturnType.element != _typeProvider.futureType.element) {
-      return _typeProvider.futureType.substitute4(<DartType>[staticReturnType]);
+    if (staticReturnType != null && _enclosingFunction.isAsynchronous) {
+      return _typeProvider.futureType.substitute4(
+          <DartType>[StaticTypeAnalyzer.flattenFutures(_typeProvider, staticReturnType)]);
     }
     return staticReturnType;
   }
 
   /**
    * Return the error code that should be used when the given class references itself directly.
    *
    * @param classElt the class that references itself
    * @return the error code that should be used
    */
@@ skipping 427 matching lines @@
     toCheck.add(type.element);
     // type arguments
     if (type is InterfaceType) {
       InterfaceType interfaceType = type;
       for (DartType typeArgument in interfaceType.typeArguments) {
         _addTypeToCheck(typeArgument);
       }
     }
   }
 }