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.
 
 @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) {
+    _doThrowNew(assertionStart, assertionEnd, message);
+  }
+
+  static _doThrowNew(int assertionStart, int assertionEnd, Object message)
       native "AssertionError_throwNew";
 
-  static void _checkAssertion(condition, int start, int end) {
+  static _checkAssertion(condition) {
     if (condition is Function) {
       condition = condition();
     }
-    if (!condition) {
-      _throwNew(start, end);
-    }
+    return condition;
   }
 
-  static void _checkConstAssertion(bool condition, int start, int end) {
-    if (!condition) {
-      _throwNew(start, end);
-    }
+  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";
   }
   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._errorMsg)
-      : super._create("is assignable", url, line, column);
+  _TypeError._create(String url, int line, int column, String errorMsg)
+      : super._create("is assignable", url, line, column, errorMsg);
 
   static _throwNew(int location,
                    Object src_value,
                    _Type dst_type,
                    String dst_name,
                    String bound_error_msg)
       native "TypeError_throwNew";
 
   static _throwNewIfNotLoaded(_LibraryPrefix prefix,
                               int location,
                               Object src_value,
                               _Type dst_type,
                               String dst_name,
                               String bound_error_msg) {
     if (!prefix.isLoaded()) {
       _throwNew(location, src_value, dst_type, dst_name, bound_error_msg);
     }
   }
 
-  String toString() => _errorMsg;
-
-  final String _errorMsg;
+  String toString() => super.message;
 }
 
 class _CastError extends Error implements CastError {
   _CastError._create(this._url, this._line, this._column, this._errorMsg);
 
   // A CastError is allocated by TypeError._throwNew() when dst_name equals
   // Symbols::InTypeCast().
 
   String toString() => _errorMsg;
 
@@ skipping 262 matching lines @@
     return msg_buf.toString();
   }
 }
 
 
 class _CompileTimeError extends Error {
   final String _errorMsg;
   _CompileTimeError(this._errorMsg);
   String toString() => _errorMsg;
 }