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

pkg/compiler/lib/src/commandline_options.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.cmdline.options;
 
 /// Commandline flags used in `dart2js.dart` and/or `apiimpl.dart`.
 class Flags {
   static const String allowMockCompilation = '--allow-mock-compilation';
   static const String allowNativeExtensions = '--allow-native-extensions';
   static const String analyzeAll = '--analyze-all';
   static const String analyzeMain = '--analyze-main';
   static const String analyzeOnly = '--analyze-only';
   static const String analyzeSignaturesOnly = '--analyze-signatures-only';
   static const String disableInlining = '--disable-inlining';
   static const String disableDiagnosticColors = '--disable-diagnostic-colors';
   static const String disableNativeLiveTypeAnalysis =
       '--disable-native-live-type-analysis';
   static const String disableTypeInference = '--disable-type-inference';
   static const String dumpInfo = '--dump-info';
   static const String enableAssertMessage = '--assert-message';
   static const String enableCheckedMode = '--enable-checked-mode';
+  static const String enableAsserts = '--enable-asserts';
   static const String enableDiagnosticColors = '--enable-diagnostic-colors';
   static const String enableExperimentalMirrors =
       '--enable-experimental-mirrors';
   static const String experimentalTrackAllocations =
       '--experimental-track-allocations';
   static const String experimentalAllocationsPath =
       '--experimental-allocations-path';
   static const String fastStartup = '--fast-startup';
   static const String fatalWarnings = '--fatal-warnings';
   static const String generateCodeWithCompileTimeErrors =
@@ skipping 60 matching lines @@
 }
 
 class Option {
   static const String showPackageWarnings =
       '${Flags.showPackageWarnings}|${Flags.showPackageWarnings}=.*';
 
   // Experimental options.
   static const String resolutionInput = '--resolution-input=.+';
   static const String bazelPaths = '--bazel-paths=.+';
 }