Add command-line parameter enabling assert messages.

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index a2f98b7b551aae6ffe08786be5d4299a079204cd..6a94b32bf79d164e3497798fe77908658aacea4f 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -355,6 +355,7 @@ Future<api.CompilationResult> compile(List<String> argv) {
     new OptionHandler('--show-package-warnings', passThrough),
     new OptionHandler('--csp', passThrough),
     new OptionHandler('--enable-experimental-mirrors', passThrough),
+    new OptionHandler('--enable-assert-message', passThrough),
     new OptionHandler('--enable-async', (_) {
       diagnosticHandler.info(
           "Option '--enable-async' is no longer needed. "
@@ -384,7 +385,7 @@ Future<api.CompilationResult> compile(List<String> argv) {
     }),
     new OptionHandler('.*', (String argument) {
       arguments.add(nativeToUriPath(argument));
-    })
+    }),
   ];
 
   parseCommandLine(handlers, argv);