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

pkg/compiler/lib/src/dart2js.dart

 // Copyright (c) 2013, 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;
 
 import 'dart:async' show Future;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io' show exit, File, FileMode, Platform, stdin, stderr;
 
@@ skipping 332 matching lines @@
     new OptionHandler(Flags.preserveUris, passThrough),
     new OptionHandler('--force-strip=.*', setStrip),
     new OptionHandler(Flags.disableDiagnosticColors, (_) {
       enableColors = false;
     }),
     new OptionHandler(Flags.enableDiagnosticColors, (_) {
       enableColors = true;
     }),
     new OptionHandler('--enable[_-]checked[_-]mode|--checked',
         (_) => setCheckedMode(Flags.enableCheckedMode)),
+    new OptionHandler(Flags.enableAsserts, passThrough),
     new OptionHandler(Flags.trustTypeAnnotations,
         (_) => setTrustTypeAnnotations(Flags.trustTypeAnnotations)),
     new OptionHandler(Flags.trustPrimitives,
         (_) => setTrustPrimitives(Flags.trustPrimitives)),
     new OptionHandler(
         Flags.trustJSInteropTypeAnnotations,
         (_) => setTrustJSInteropTypeAnnotations(
             Flags.trustJSInteropTypeAnnotations)),
     new OptionHandler(r'--help|/\?|/h', (_) => wantHelp = true),
     new OptionHandler('--packages=.+', setPackageConfig),
@@ skipping 717 matching lines @@
   @override
   void add(String event) {
     sb.write(event);
   }
 
   @override
   void close() {
     // Do nothing.
   }
 }