dart2Js: bailout early when using the new emitter with mirrors.

pkg/compiler/lib/src/warnings.dart

Index: pkg/compiler/lib/src/warnings.dart
diff --git a/pkg/compiler/lib/src/warnings.dart b/pkg/compiler/lib/src/warnings.dart
index 6a78b8adff1a26d5283a59e0f4e2252c52ed21cf..61a20761eda2628dd56a782ec46662ba24f8a0ec 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -2382,6 +2382,12 @@ $IMPORT_EXPERIMENTAL_MIRRORS_PADDING#{importChain}
 ****************************************************************
 ''');
 
+
+  static const MessageKind MIRRORS_LIBRARY_NEW_EMITTER =
+      const MessageKind(
+          "dart:mirrors library is not supported when using the new emitter "
+            "(DART_VM_OPTIONS='-Ddart2js.use.new.emitter=true')");
+
   static const MessageKind CALL_NOT_SUPPORTED_ON_NATIVE_CLASS =
       const MessageKind(
           "Non-supported 'call' member on a native class, or a "