Flatten _Future at runtime as well

lib/src/compiler/code_generator.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.
 
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart' show Token, TokenType;
@@ skipping 807 matching lines @@
 
     return _statement(result);
   }
 
   /// Wraps a possibly generic class in its type arguments.
   JS.Statement _defineClassTypeArguments(TypeDefiningElement element,
       List<TypeParameterElement> formals, JS.Statement body) {
     assert(formals.isNotEmpty);
     var genericCall = js.call('dart.generic((#) => { #; return #; })',
         [_emitTypeFormals(formals), body, element.name]);
-    if (element.type.isDartAsyncFuture) {
+    if (element.library.isDartAsync &&
+        (element.name == "Future" || element.name == "_Future")) {
       genericCall = js.call('dart.flattenFutures(#)', [genericCall]);
     }
     var genericDef = js.statement(
         '# = #;', [_emitTopLevelName(element, suffix: r'$'), genericCall]);
     var dynType = fillDynamicTypeArgs(element.type);
     var genericInst = _emitType(dynType, lowerGeneric: true);
     return js.statement(
         '{ #; # = #; }', [genericDef, _emitTopLevelName(element), genericInst]);
   }
 
@@ skipping 3696 matching lines @@
 }
 
 bool isLibraryPrefix(Expression node) =>
     node is SimpleIdentifier && node.staticElement is PrefixElement;
 
 LibraryElement _getLibrary(AnalysisContext c, String uri) =>
     c.computeLibraryElement(c.sourceFactory.forUri(uri));
 
 bool _isDartRuntime(LibraryElement l) =>
     l.isInSdk && l.source.uri.toString() == 'dart:_runtime';