Update assert to follow the spec.

compiler/lib/implementation/core.js

 // Copyright (c) 2011, 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.
 
 /**
  * Helpers for lazy static initialization.
  */
 var static$uninitialized = {};
 var static$initializing = {};
 
@@ skipping 163 matching lines @@
 /**
  * Dart null object that should be used by JS implementation to test for
  * Dart null.
  *
  * TODO(ngeoffray): update dartc to generate this variable instead of
  *                  undefined.
  * @const
  */
 var $Dart$Null = void 0;
 
-function assert(expr, msg) {
-  var val = typeof(expr) == 'function' ? expr() : expr;
-  if (!val) {
-    // TODO: throw a Dart AssertionError instead
-    var err = new Error('Assertion failed. ' + (msg || ''));
-    Error.captureStackTrace && Error.captureStackTrace(err);
-    throw err;
+function assert(expr) {
+  var val = typeof(expr) == 'function' ? $dartcall(expr, []) : expr;
+  if (val !== true) {
+    $Dart$ThrowException(native_ExceptionHelper_createAssertionError());
   }
 }
 
 // TODO(jimhug): Remove these functions after updating compiler backend.
 function BIT_OR$operator(val1, val2) {
   return (typeof(val1) == 'number' && typeof(val2) == 'number')
       ? val1 | val2
       : val1.BIT_OR$operator(val2);
 }
 
@@ skipping 265 matching lines @@
     return match;
   }
   o.$dartConstId = id;
   $consts[key] = o;
   return o;
 }
 
 function $Dart$MapLiteralFactory() {
   return native__CoreJsUtil__newMapLiteral();
 }