Add better error reporting to utils tests.

tests/utils/source_mirrors_test.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.
 // VMOptions=
 // VMOptions=--print-object-histogram
 
 // Smoke test of the dart2js compiler API.
 library source_mirrors_test;
 
 import 'dart:async';
@@ skipping 12 matching lines @@
   result.then((mirrorSystem) {
     if (mirrorSystem == null) {
       throw 'Analysis failed';
     }
     mirrorSystem.libraries.forEach((uri, library) {
       print(library);
       library.declarations.forEach((name, declaration) {
         print(' $name:$declaration');
       });
     });
-  }, onError: (e) {
-      throw 'Analysis failed';
+  }, onError: (e, s) {
+      throw 'Analysis failed: $e\n$s';
   }).then(asyncSuccess);
 }