Revert r42145 to fix co19 tests

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;
 import 'dart:convert' show JSON;
 
 patch class Error {
   /* patch */ static String _objectToString(Object object) {
     return Object._toString(object);
   }
 
   /* patch */ static String _stringToSafeString(String string) {
     return JSON.encode(string);
   }
 
   /* patch */ StackTrace get stackTrace => _stackTrace;
 
   StackTrace _stackTrace;
 }
 
 class _AssertionError extends Error implements AssertionError {
   _AssertionError._create(
       this._failedAssertion, this._url, this._line, this._column);
 
   static _throwNew(int assertionStart, int assertionEnd)
       native "AssertionError_throwNew";
 
   String toString() {
-    if (_url == null) {
-      return _failedAssertion;
-    }
     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;
@@ skipping 302 matching lines @@
   String toString() => "Javascript Integer Overflow: $_value";
 }
 
 class _JavascriptCompatibilityError extends Error {
   final String _msg;
 
   _JavascriptCompatibilityError(this._msg);
   String toString() => "Javascript Compatibility Error: $_msg";
 }