Fix handling of async patches.

sdk/lib/_internal/js_runtime/lib/isolate_patch.dart

 // Copyright (c) 2012, 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.
 
 // Patch file for the dart:isolate library.
 
 import 'dart:_js_helper' show patch;
 import 'dart:_isolate_helper'
     show CapabilityImpl, IsolateNatives, ReceivePortImpl, RawReceivePortImpl;
 
@@ skipping 14 matching lines @@
   }
 
   @patch
   static Future<Uri> get packageConfig {
     throw new UnsupportedError("Isolate.packageConfig");
   }
 
   static Uri _packageBase = Uri.base.resolve(IsolateNatives.packagesBase);
 
   @patch
-  static Future<Uri> resolvePackageUri(Uri packageUri) async {
-    if (packageUri.scheme != 'package') return packageUri;
-    return _packageBase.resolveUri(packageUri.replace(scheme: ''));
+  static Future<Uri> resolvePackageUri(Uri packageUri) {
+    if (packageUri.scheme != 'package') {
+      return new Future<Uri>.value(packageUri);
+    }
+    return new Future<Uri>.value(
+        _packageBase.resolveUri(packageUri.replace(scheme: '')));
   }
 
   @patch
   static Future<Isolate> spawn(void entryPoint(message), var message,
       {bool paused: false,
       bool errorsAreFatal,
       SendPort onExit,
       SendPort onError}) {
     bool forcePause =
         (errorsAreFatal != null) || (onExit != null) || (onError != null);
@@ skipping 185 matching lines @@
   factory RawReceivePort([void handler(event)]) {
     return new RawReceivePortImpl(handler);
   }
 }
 
 @patch
 class Capability {
   @patch
   factory Capability() = CapabilityImpl;
 }