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

pkg/compiler/lib/src/options.dart

 // Copyright (c) 2016, 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.src.options;
 
 import '../compiler.dart' show PackagesDiscoveryProvider;
 import 'commandline_options.dart' show Flags;
 
 /// Options used for controlling diagnostic messages.
@@ skipping 278 matching lines @@
             _extractOptionalCsvOption(options, Flags.showPackageWarnings),
         disableInlining: _hasOption(options, Flags.disableInlining),
         disableTypeInference: _hasOption(options, Flags.disableTypeInference),
         dumpInfo: _hasOption(options, Flags.dumpInfo),
         enableExperimentalMirrors:
             _hasOption(options, Flags.enableExperimentalMirrors),
         enableMinification: _hasOption(options, Flags.minify),
         enableNativeLiveTypeAnalysis:
             !_hasOption(options, Flags.disableNativeLiveTypeAnalysis),
         enableTypeAssertions: _hasOption(options, Flags.enableCheckedMode),
-        enableUserAssertions: _hasOption(options, Flags.enableCheckedMode),
+        enableUserAssertions: _hasOption(options, Flags.enableCheckedMode) ||
+            _hasOption(options, Flags.enableAsserts),
         experimentalTrackAllocations:
             _hasOption(options, Flags.experimentalTrackAllocations),
         experimentalAllocationsPath: _extractStringOption(
             options, "${Flags.experimentalAllocationsPath}=", null),
         generateCodeWithCompileTimeErrors:
             _hasOption(options, Flags.generateCodeWithCompileTimeErrors),
         generateSourceMap: !_hasOption(options, Flags.noSourceMaps),
         kernelGlobalInference: _hasOption(options, Flags.kernelGlobalInference),
         outputUri: _extractUriOption(options, '--out='),
         platformConfigUri:
@@ skipping 436 matching lines @@
       _extractStringOption(options, "--platform-config=", null),
       _extractCsvOption(options, '--categories='));
 }
 
 /// Locations of the platform descriptor files relative to the library root.
 const String _clientPlatform = "lib/dart_client.platform";
 const String _serverPlatform = "lib/dart_server.platform";
 const String _sharedPlatform = "lib/dart_shared.platform";
 
 const String _UNDETERMINED_BUILD_ID = "build number could not be determined";