Move dart2jslib parts into separate libraries.

pkg/compiler/lib/src/diagnostic_listener.dart

Index: pkg/compiler/lib/src/diagnostic_listener.dart
diff --git a/pkg/compiler/lib/src/diagnostic_listener.dart b/pkg/compiler/lib/src/diagnostic_listener.dart
index d57b11d0d54bf7aae617e02f1094ae4122642f75..96d45eeca2b2e4f656a526b94a88c0712d4aa3e8 100644
--- a/pkg/compiler/lib/src/diagnostic_listener.dart
+++ b/pkg/compiler/lib/src/diagnostic_listener.dart
@@ -2,7 +2,15 @@
 // 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 dart2js;
+library dart2js.diagnostic_listener;
+
+import 'dart2jslib.dart' show
+    SourceSpan;
+import 'elements/elements.dart' show
+    Element;
+import 'messages.dart';
+import 'util/util.dart' show
+    Spannable;
 
 abstract class DiagnosticListener {
   // TODO(karlklose): rename log to something like reportInfo.