Add --assert-message option.

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, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 338 matching lines @@
     new OptionHandler('--categories=.*', setCategories),
     new OptionHandler(Flags.disableTypeInference, implyCompilation),
     new OptionHandler(Flags.terse, passThrough),
     new OptionHandler('--deferred-map=.+', implyCompilation),
     new OptionHandler(Flags.dumpInfo, setDumpInfo),
     new OptionHandler('--disallow-unsafe-eval',
                       (_) => hasDisallowUnsafeEval = true),
     new OptionHandler(Flags.showPackageWarnings, passThrough),
     new OptionHandler(Flags.useContentSecurityPolicy, passThrough),
     new OptionHandler(Flags.enableExperimentalMirrors, passThrough),
+    new OptionHandler(Flags.enableAssertMessage, passThrough),
     new OptionHandler('--enable-async', (_) {
       diagnosticHandler.info(
           "Option '--enable-async' is no longer needed. "
           "Async-await is supported by default.",
           api.Diagnostic.HINT);
     }),
     new OptionHandler('--enable-null-aware-operators',  (_) {
       diagnosticHandler.info(
           "Option '--enable-null-aware-operators' is no longer needed. "
           "Null aware operators are supported by default.",
@@ skipping 399 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }