Allow return of Future in async function

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 /// General type checking tests
 library test.src.task.strong.checker_test;
 
 import 'package:unittest/unittest.dart';
@@ skipping 2346 matching lines @@
     testChecker('async', {
       '/main.dart': '''
         import 'dart:async';
         import 'dart:math' show Random;
 
         dynamic x;
 
         foo1() async => x;
         Future foo2() async => x;
         Future<int> foo3() async => (/*info:DynamicCast*/x);
-        Future<int> foo4() async => (/*severe:StaticTypeError*/new Future<int>.value(/*info:DynamicCast*/x));
+        Future<int> foo4() async => (new Future<int>.value(/*info:DynamicCast*/x));
+        Future<int> foo5() async => (/*severe:StaticTypeError*/new Future<String>.value(/*info:DynamicCast*/x));
 
         bar1() async { return x; }
         Future bar2() async { return x; }
         Future<int> bar3() async { return (/*info:DynamicCast*/x); }
-        Future<int> bar4() async { return (/*severe:StaticTypeError*/new Future<int>.value(/*info:DynamicCast*/x)); }
+        Future<int> bar4() async { return (new Future<int>.value(/*info:DynamicCast*/x)); }
+        Future<int> bar5() async { return (/*severe:StaticTypeError*/new Future<String>.value(/*info:DynamicCast*/x)); }
 
         int y;
         Future<int> z;
 
         void baz() async {
           int a = /*info:DynamicCast*/await x;
           int b = await y;
           int c = await z;
           String d = /*severe:StaticTypeError*/await z;
         }
 
         Future<bool> get issue_264 async {
           await 42;
           if (new Random().nextBool()) {
             return true;
           } else {
-            return /*severe:StaticTypeError*/new Future<bool>.value(false);
+            return new Future<bool>.value(false);
           }
         }
     '''
     });
 
     testChecker('async*', {
       '/main.dart': '''
         import 'dart:async';
 
         dynamic x;
@@ skipping 24 matching lines @@
 
         baz1() sync* { yield* (/*info:DynamicCast*/x); }
         Iterable baz2() sync* { yield* (/*info:DynamicCast*/x); }
         Iterable<int> baz3() sync* { yield* (/*warning:DownCastComposite*/x); }
         Iterable<int> baz4() sync* { yield* new Iterable<int>(); }
         Iterable<int> baz5() sync* { yield* (/*info:InferredTypeAllocation*/new Iterable()); }
     '''
     });
   });
 }