Allow plugins to provide completions for non-dart files

pkg/analysis_server/test/integration/completion/get_suggestions_test.dart

 // Copyright (c) 2014, 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.
 
 import 'package:analysis_server/protocol/protocol_generated.dart';
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../support/integration_tests.dart';
@@ skipping 100 matching lines @@
     //   writeFile(pathname, text);
     // Don't wait for any results except the completion notifications
     standardAnalysisSetup(subscribeStatus: false);
     // Missing file and no overlay
     //sendAnalysisUpdateContent({path: new AddContentOverlay(content)});
     var errorToken = 'exception from server';
     return sendCompletionGetSuggestions(path, 0).catchError((e) {
       // Exception expected
       return errorToken;
     }).then((result) {
-      expect(result, same(errorToken));
+      expect(result, new isInstanceOf<CompletionGetSuggestionsResult>());
     });
   }
 }