Implement `FutureOr<T>` for dart2js.

pkg/compiler/lib/src/resolution/type_resolver.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.
 
 library dart2js.resolution.types;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
 import '../dart_types.dart';
 import '../elements/elements.dart'
@@ skipping 136 matching lines @@
       if (element is ErroneousElement) {
         type = reportFailureAndCreateType(
             element.messageKind, element.messageArguments,
             erroneousElement: element);
       } else {
         type = const DynamicType();
       }
     } else if (!element.impliesType) {
       type = reportFailureAndCreateType(
           MessageKind.NOT_A_TYPE, {'node': node.typeName});
+    } else if (element.library.isPlatformLibrary &&
+        element.name == 'FutureOr') {
+      type = const DynamicType();
+      registry.useType(node, type);
+      return type;
     } else {
       bool addTypeVariableBoundsCheck = false;
       if (element.isClass) {
         ClassElement cls = element;
         // TODO(johnniwinther): [ensureClassWillBeResolvedInternal] should imply
         // [computeType].
         resolver.ensureClassWillBeResolvedInternal(cls);
         cls.computeType(resolution);
         List<DartType> arguments = <DartType>[];
         bool hasTypeArgumentMismatch =
@@ skipping 108 matching lines @@
       arguments.add(argType);
     }
     if (index < expectedVariables) {
       reporter.reportWarningMessage(
           node.typeArguments, MessageKind.MISSING_TYPE_ARGUMENT);
       typeArgumentCountMismatch = true;
     }
     return typeArgumentCountMismatch;
   }
 }