Change assert implementation to not depend on a top-level function called 'assert'.

tests/compiler/dart2js/unparser_test.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:uri');
 #import('parser_helper.dart');
 #import('mock_compiler.dart');
 #import("../../../lib/compiler/compiler.dart");
 #import("../../../lib/compiler/implementation/leg.dart", prefix:'leg');
 #import("../../../lib/compiler/implementation/dart_backend/dart_backend.dart");
@@ skipping 17 matching lines @@
 interface num {}
 interface int extends num {}
 interface double extends num {}
 interface String {}
 interface Function {}
 interface List {}
 interface Map {}
 interface Closure {}
 interface Dynamic {}
 interface Null {}
-assert() {}
 class Math {
   static double parseDouble(String s) => 1.0;
 }
 ''';
 
 const ioLib = @'''
 #library('io');
 class Platform {
   static int operatingSystem;
 }
@@ skipping 801 matching lines @@
   testDoubleMains();
   testInterfaceDefaultAnotherLib();
   testStaticAccessIoLib();
   testLocalFunctionPlaceholder();
   testMinification();
   testClosureLocalsMinified();
   testParametersMinified();
   testTypeVariablesInDifferentLibraries();
   testDeclarationTypePlaceholders();
 }