Various infrastructure changes.

runtime/lib/errors_patch.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.
 
 @patch
 class Error {
   @patch
   static String _objectToString(Object object) {
     return Object._toString(object);
   }
@@ skipping 395 matching lines @@
 }
 
 dynamic _classIdEqualsAssert(
     int position, dynamic instance, _Type type, int cid, int otherCid) {
   if (cid != otherCid && instance != null) {
     _TypeError._throwNew(position, instance, type, " in type cast", null);
   }
 
   return instance;
 }
+
+/// Used by Fasta to report a runtime error when a final field with an
+/// initializer is also initialized in a generative constructor.
+///
+/// Note: in strong mode, this is a compile-time error and this class becomes
+/// obsolete.
+class _DuplicatedFieldInitializerError extends Error {
+  final String _name;
+
+  _DuplicatedFieldInitializerError(this._name);
+
+  toString() => "Error: field '$_name' is already initialized.";
+}