Fix http://dartbug.com/21335

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.
 
 import 'dart:mirrors' show MirrorSystem;
 
 patch class Error {
   /* patch */ static String _objectToString(Object object) {
     return Object._toString(object);
   }
 
   /* patch */ StackTrace get stackTrace => _stackTrace;
 
   StackTrace _stackTrace;
 }
 
-patch class AssertionError extends Error {
-  AssertionError._create(
+class _AssertionError extends Error implements AssertionError {
+  _AssertionError._create(

[Lasse Reichstein Nielsen, 2014/10/21 07:53:43]

This constructor doesn't even need to be named any more.
Change it only if you actually care :)

       this._failedAssertion, this._url, this._line, this._column);
 
   static _throwNew(int assertionStart, int assertionEnd)
       native "AssertionError_throwNew";
 
   String toString() {
     var columnInfo = "";
     if (_column > 0) {
       // Only add column information if it is valid.
       columnInfo = " pos $_column";
     }
     return "'$_url': Failed assertion: line $_line$columnInfo: "
         "'$_failedAssertion' is not true.";
   }
   final String _failedAssertion;
   final String _url;
   final int _line;
   final int _column;
 }
 
-patch class TypeError extends AssertionError {
-  TypeError._create(String url, int line, int column,
-                    this._srcType, this._dstType, this._dstName,
-                    this._errorMsg)
+class _TypeError extends _AssertionError implements TypeError {
+  _TypeError._create(String url, int line, int column,
+                     this._srcType, this._dstType, this._dstName,
+                     this._errorMsg)
       : super._create("is assignable", url, line, column);
 
   static _throwNew(int location,
                    Object src_value,
                    String dst_type_name,
                    String dst_name,
                    String error_msg)
       native "TypeError_throwNew";
 
   String toString() {
     String str = (_errorMsg != null) ? _errorMsg : "";
     if ((_dstName != null) && (_dstName.length > 0)) {
       str = "${str}type '$_srcType' is not a subtype of "
             "type '$_dstType' of '$_dstName'.";
     } else {
       str = "${str}type error.";
     }
     return str;
   }
 
   final String _srcType;
   final String _dstType;
   final String _dstName;
   final String _errorMsg;
 }
 
-patch class CastError extends Error {
-  CastError._create(this._url, this._line, this._column,
-                    this._srcType, this._dstType, this._dstName,
-                    this._errorMsg);
+class _CastError extends Error implements CastError {
+  _CastError._create(this._url, this._line, this._column,
+                     this._srcType, this._dstType, this._dstName,
+                     this._errorMsg);
 
   // A CastError is allocated by TypeError._throwNew() when dst_name equals
   // Exceptions::kCastErrorDstName.
 
   String toString() {
     String str = (_errorMsg != null) ? _errorMsg : "";
     str = "${str}type '$_srcType' is not a subtype of "
           "type '$_dstType' in type cast.";
     return str;
   }
@@ skipping 254 matching lines @@
   String toString() => "Javascript Integer Overflow: $_value";
 }
 
 class _JavascriptCompatibilityError extends Error {
   final String _msg;
 
   _JavascriptCompatibilityError(this._msg);
   String toString() => "Javascript Compatibility Error: $_msg";
 }