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

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 5427478abab9278d3dc11d409734ffd150fd40d9..48f82b9dce2237e0de4cad442b1ba3a32b81511b 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -1322,10 +1322,16 @@ abstract class Compiler implements DiagnosticListener {
           }
           return true;
         });
-        reportWarning(NO_LOCATION_SPANNABLE,
-            MessageKind.IMPORT_EXPERIMENTAL_MIRRORS,
-            {'importChain': importChains.join(
-                 MessageKind.IMPORT_EXPERIMENTAL_MIRRORS_PADDING)});
+
+        if (const bool.fromEnvironment("dart2js.use.new.emitter")) {
+          reportError(NO_LOCATION_SPANNABLE,
+                      MessageKind.MIRRORS_LIBRARY_NEW_EMITTER);
+        } else {
+          reportWarning(NO_LOCATION_SPANNABLE,
+             MessageKind.IMPORT_EXPERIMENTAL_MIRRORS,
+              {'importChain': importChains.join(
+                   MessageKind.IMPORT_EXPERIMENTAL_MIRRORS_PADDING)});
+        }
       }
 
       functionClass.ensureResolved(this);