Add optional message argument to assert statements in the VM.

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:_internal' as internal;
 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);
+      this._failedAssertion, this._url, this._line, this._column,
+      this.message);
 
-  static _throwNew(int assertionStart, int assertionEnd)
+  static _throwNew(int assertionStart, int assertionEnd, Object message)
       native "AssertionError_throwNew";
 
+  String get _messageString {
+    if (message == null) return "is not true.";
+    if (message is String) return message;
+    return Error.safeToString(message);
+  }
+
   String toString() {
     if (_url == null) {
-      return _failedAssertion;
+      if (message == null) return _failedAssertion;
+      return "'$_failedAssertion' $_messageString";
     }
     var columnInfo = "";
     if (_column > 0) {
       // Only add column information if it is valid.
       columnInfo = " pos $_column";
     }
     return "'$_url': Failed assertion: line $_line$columnInfo: "
-        "'$_failedAssertion' is not true.";
+           "'$_failedAssertion' $_messageString";

[Lasse Reichstein Nielsen, 2015/08/28 18:51:59]

The format is negotiable.

   }
   final String _failedAssertion;
   final String _url;
   final int _line;
   final int _column;
+  final Object message;
 }
 
 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);
+                     String error_msg)
+      : super._create("is assignable", url, line, column, error_msg);
 
   static _throwNew(int location,
                    Object src_value,
                    String dst_type_name,
                    String dst_name,
                    String error_msg)
       native "TypeError_throwNew";
 
   static _throwNewIfNotLoaded(_LibraryPrefix prefix,
                               int location,
                               Object src_value,
                               String dst_type_name,
                               String dst_name,
                               String error_msg) {
     if (!prefix.isLoaded()) {
       _throwNew(location, src_value, dst_type_name, dst_name, error_msg);
     }
   }
 
 
   String toString() {
-    String str = (_errorMsg != null) ? _errorMsg : "";
+    String str = (message != null) ? message : "";
     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;
 }
 
 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.
 
@@ skipping 273 matching lines @@
   String toString() => "Javascript Integer Overflow: $_value";
 }
 
 class _JavascriptCompatibilityError extends Error {
   final String _msg;
 
   _JavascriptCompatibilityError(this._msg);
   String toString() => "Javascript Compatibility Error: $_msg";
 }