Add location information to watchers to make them more debuggable.

lib/src/compiler.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.
 
 library compiler;
 
 import 'dart:async';
 import 'dart:collection' show SplayTreeMap;
 import 'dart:json' as json;
 import 'package:analyzer_experimental/src/generated/ast.dart' show Directive, UriBasedDirective;
@@ skipping 794 matching lines @@
 }
 
 /**
  * The code that will be used to bootstrap the application, this is inlined in
  * the main.html.html output file.
  */
 String _bootstrapCode(String userMainImport, bool useObservers) => """
 library bootstrap;
 
 import 'package:web_ui/watcher.dart' as watcher;
+import 'package:logging/logging.dart';
 import '$userMainImport' as userMain;
 
 main() {
   watcher.useObservers = $useObservers;
+  Logger.root.onRecord.listen((record) {
+    print('\${record.level.name.toLowerCase()}: \${record.message}');
+  });
   userMain.main();
   userMain.init_autogenerated();
 }
 """;