Improve the error message when an import fails.

runtime/bin/vmservice/loader.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.
 
 part of vmservice_io;
 
 _log(msg) {
   print("% $msg");
 }
 
@@ skipping 39 matching lines @@
   Timer.run(() {});
 }
 
 void _loadFile(SendPort sp, int id, Uri uri) {
   var path = uri.toFilePath();
   var sourceFile = new File(path);
   sourceFile.readAsBytes().then((data) {
     _sendResourceResponse(sp, id, data);
   },
   onError: (e) {
-    var err = "Error loading $uri:\n  $e";
-    _sendResourceResponse(sp, id, err);
+    _sendResourceResponse(sp, id, e.toString());
   });
 }
 
 void _loadDataUri(SendPort sp, int id, Uri uri) {
   try {
     var mime = uri.data.mimeType;
     if ((mime != "application/dart") &&
         (mime != "text/plain")) {
       throw "MIME-type must be application/dart or text/plain: $mime given.";
     }
@@ skipping 382 matching lines @@
   assert(id != null);
   String resource = request[3];
   assert(resource != null);
   var uri = Uri.parse(resource);
   if (id >= 0) {
     _handleResourceRequest(sp, traceLoading, id, uri);
   } else {
     _handlePackagesRequest(sp, traceLoading, id, uri);
   }
 }