Add location information to watchers to make them more debuggable.

lib/src/compiler.dart

Index: lib/src/compiler.dart
diff --git a/lib/src/compiler.dart b/lib/src/compiler.dart
index 0404014bcc34dcd33b6fcda76affdf70a411daa5..65a70e7eded569fa5678579d2be2e7008d4de754 100644
--- a/lib/src/compiler.dart
+++ b/lib/src/compiler.dart
@@ -812,10 +812,14 @@ 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();
 }