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

tools/testing/dart/configuration.dart

 // Copyright (c) 2017, 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:convert';
 import 'dart:io';
 
 import 'compiler_configuration.dart';
 import 'http_server.dart';
@@ skipping 29 matching lines @@
       this.isVerbose,
       this.listTests,
       this.printTiming,
       this.printReport,
       this.reportInJson,
       this.resetBrowser,
       this.skipCompilation,
       this.useBlobs,
       this.useSdk,
       this.useFastStartup,
+      this.useEnableAsserts,
       this.useDart2JSWithKernel,
       this.writeDebugLog,
       this.writeTestOutcomeLog,
       this.drtPath,
       this.dartiumPath,
       this.chromePath,
       this.safariPath,
       this.firefoxPath,
       this.dartPath,
       this.dartPrecompiledPath,
@@ skipping 42 matching lines @@
   final bool isVerbose;
   final bool listTests;
   final bool printTiming;
   final bool printReport;
   final bool reportInJson;
   final bool resetBrowser;
   final bool skipCompilation;
   final bool useBlobs;
   final bool useSdk;
   final bool useFastStartup;
+  final bool useEnableAsserts;
   final bool useDart2JSWithKernel;
   final bool writeDebugLog;
   final bool writeTestOutcomeLog;
 
   // Various file paths.
 
   final String drtPath;
   final String dartiumPath;
   final String chromePath;
   final String safariPath;
@@ skipping 86 matching lines @@
     if (isChecked) args.add('--enable-checked-mode');
 
     if (!runtime.isBrowser) {
       args.add("--allow-mock-compilation");
       args.add("--categories=all");
     }
 
     if (isMinified) args.add("--minify");
     if (isCsp) args.add("--csp");
     if (useFastStartup) args.add("--fast-startup");
+    if (useEnableAsserts) args.add("--enable-asserts");
     if (useDart2JSWithKernel) args.add("--use-kernel");
     return args;
   }
 
   String _windowsSdkPath;
   String get windowsSdkPath {
     if (!Platform.isWindows) {
       throw new StateError(
           "Should not use windowsSdkPath when not running on Windows.");
     }
@@ skipping 524 matching lines @@
 
       case macos:
         return 'xcodebuild/';
     }
 
     throw "unreachable";
   }
 
   String toString() => "System($name)";
 }