dart2js: provide a better error-message, when a backend doesn't support mirrors.

pkg/compiler/lib/src/compiler.dart

 // Copyright (c) 2012, 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.compiler_base;
 
 import 'dart:async' show
     EventSink,
     Future;
 
@@ skipping 853 matching lines @@
               {'uri': uri,
                'importChain': importChains.join(
                    MessageTemplate.DISALLOWED_LIBRARY_IMPORT_PADDING)});
         }
       }
 
       if (!loadedLibraries.containsLibrary(Uris.dart_core)) {
         return null;
       }
 
-      if (!enableExperimentalMirrors &&
-          loadedLibraries.containsLibrary(Uris.dart_mirrors)) {
+      bool importsMirrorsLibrary =
+          loadedLibraries.containsLibrary(Uris.dart_mirrors);
+      if (importsMirrorsLibrary && !backend.supportsReflection) {
         Set<String> importChains =
             computeImportChainsFor(loadedLibraries, Uris.dart_mirrors);
-        if (!backend.supportsReflection) {
-          reportErrorMessage(
-              NO_LOCATION_SPANNABLE,
-              MessageKind.MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND);
-        } else {
-          reportWarningMessage(
-              NO_LOCATION_SPANNABLE,
-              MessageKind.IMPORT_EXPERIMENTAL_MIRRORS,
-              {'importChain': importChains.join(
-                   MessageTemplate.IMPORT_EXPERIMENTAL_MIRRORS_PADDING)});
-        }
+        reportErrorMessage(
+            NO_LOCATION_SPANNABLE,
+            MessageKind.MIRRORS_LIBRARY_NOT_SUPPORT_BY_BACKEND,
+            {'importChain': importChains.join(
+                MessageTemplate.MIRRORS_NOT_SUPPORTED_BY_BACKEND_PADDING)});
+      } else if (importsMirrorsLibrary && !enableExperimentalMirrors) {
+        Set<String> importChains =
+            computeImportChainsFor(loadedLibraries, Uris.dart_mirrors);
+        reportWarningMessage(
+            NO_LOCATION_SPANNABLE,
+            MessageKind.IMPORT_EXPERIMENTAL_MIRRORS,
+            {'importChain': importChains.join(
+                 MessageTemplate.IMPORT_EXPERIMENTAL_MIRRORS_PADDING)});
       }
 
       functionClass.ensureResolved(this);
       functionApplyMethod = functionClass.lookupLocalMember('apply');
 
       if (preserveComments) {
         return libraryLoader.loadLibrary(Uris.dart_mirrors)
             .then((LibraryElement libraryElement) {
           documentClass = libraryElement.find('Comment');
         });
@@ skipping 951 matching lines @@
 
   @override
   InterfaceType streamType([DartType elementType]) {
     InterfaceType type = streamClass.computeType(compiler);
     if (elementType == null) {
       return streamClass.rawType;
     }
     return type.createInstantiation([elementType]);
   }
 }