Nicer message when attempting to use Observatory from dart_bootstrap, dart_precompiled_runtime or a…

runtime/lib/vmservice_patch.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.
 
 @patch
 class Asset {
   /// Call to request assets from the embedder.
   @patch
   static HashMap<String, Asset> request() {
-    HashMap<String, Asset> assets = new HashMap<String, Asset>();
     Uint8List tarBytes = _requestAssets();
     if (tarBytes == null) {
-      return assets;
+      return null;
     }
     List assetList = _decodeAssets(tarBytes);
+    HashMap<String, Asset> assets = new HashMap<String, Asset>();
     for (int i = 0; i < assetList.length; i += 2) {
       var a = new Asset(assetList[i], assetList[i + 1]);
       assets[a.name] = a;
     }
     return assets;
   }
 }
 
 List _decodeAssets(Uint8List data) native "VMService_DecodeAssets";
 
@@ skipping 13 matching lines @@
 void onServerAddressChange(String address)
     native "VMService_OnServerAddressChange";
 @patch
 bool _vmListenStream(String streamId) native "VMService_ListenStream";
 @patch
 void _vmCancelStream(String streamId) native "VMService_CancelStream";
 @patch
 Uint8List _requestAssets() native "VMService_RequestAssets";
 @patch
 void _spawnUriNotify(obj, String token) native "VMService_spawnUriNotify";