Add support to dart2js for option --enable-asserts.

tools/testing/dart/test_configurations.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:async';
 import 'dart:io';
 import 'dart:math' as math;
 
 import 'android.dart';
 import 'browser_controller.dart';
@@ skipping 86 matching lines @@
     for (var configuration in configurations) {
       var settings = [
         configuration.compiler.name,
         configuration.runtime.name,
         configuration.mode.name,
         configuration.architecture.name
       ];
       if (configuration.isChecked) settings.add('checked');
       if (configuration.isStrong) settings.add('strong');
       if (configuration.useFastStartup) settings.add('fast-startup');
+      if (configuration.useEnableAsserts) settings.add('enable-asserts');
       outputWords.add(settings.join('_'));
     }
     print(outputWords.join(' '));
   }
 
   var runningBrowserTests =
       configurations.any((config) => config.runtime.isBrowser);
 
   var serverFutures = <Future>[];
   var testSuites = <TestSuite>[];
@@ skipping 142 matching lines @@
   // Start all the HTTP servers required before starting the process queue.
   if (!serverFutures.isEmpty) {
     await Future.wait(serverFutures);
   }
 
   // [firstConf] is needed here, since the ProcessQueue needs to know the
   // settings of 'noBatch' and 'local_ip'
   new ProcessQueue(firstConf, maxProcesses, maxBrowserProcesses, startTime,
       testSuites, eventListener, allTestsFinished, verbose, adbDevicePool);
 }