dart2js: Fix check for support of reflection.

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 350af41e80894e0989a47e6dec38a3a09e18f760..e2f2fb334566857b46b9ede455933a230df0b192 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -2524,10 +2524,9 @@ $IMPORT_EXPERIMENTAL_MIRRORS_PADDING#{importChain}
 ''');
 
 
-  static const MessageKind MIRRORS_LIBRARY_NEW_EMITTER =
+  static const MessageKind MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND =
       const MessageKind(
-          "dart:mirrors library is not supported when using the new emitter "
-            "(DART_VM_OPTIONS='-Ddart2js.use.new.emitter=true')");
+          "dart:mirrors library is not supported when using this backend.");

[Johnni Winther, 2015/07/16 18:14:29]

Which backend? Can you provide more info and maybe howToFix to the user?

[floitsch, 2015/07/17 13:16:19]

I wondered. (After all I removed the old one).
I think the complexity is not worth the effort:
- the emitter knows if it supports reflection.
- the choice of how the emitter was selected is not completely clear. Currently
it's a compile-time constant. It should become a flag, and could also change
depending on different things (like using the startup-emitter for CSP
compilation if there is no reflection).

All in all, I expect this to be a rare problem.

 
   static const MessageKind CALL_NOT_SUPPORTED_ON_NATIVE_CLASS =
       const MessageKind(