(Fix #215) better error printing in editor & extension, adds mapping for editor

test/testing.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.
 
 /** Common definitions used for setting up the test environment. */
 library testing;
 
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/parser.dart';
 import 'package:web_components/src/analyzer.dart';
 import 'package:web_components/src/info.dart';
 import 'package:web_components/src/messages.dart';
 import 'package:web_components/src/options.dart';
 import 'package:web_components/src/files.dart';
 import 'package:web_components/src/file_system/path.dart';
 
 useMockMessages() {
-  messages = new Messages(printHandler: (message) {});
+  messages = new Messages(shouldPrint: false);
 }
 
 Document parseDocument(String html) => parse(html);
 
 Element parseSubtree(String html) => parseFragment(html).nodes[0];
 
 ElementInfo analyzeElement(Element elem) {
   return analyzeNodeForTesting(elem).bodyInfo;
 }
 
@@ skipping 37 matching lines @@
   return res;
 }
 
 Map<String, FileInfo> _toStringMap(Map<Path, FileInfo> map) {
   var res = new Map<String, FileInfo>();
   for (var k in map.keys) {
     res[k.toString()] = map[k];
   }
   return res;
 }