extract LibraryPrefixContributor from imported reference contributor

pkg/analysis_server/test/domain_completion_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.
 
 library test.domain.completion;
 
 import 'dart:async';
 
 import 'package:analysis_server/plugin/protocol/protocol.dart';
 import 'package:analysis_server/src/analysis_server.dart';
@@ skipping 454 matching lines @@
 
   test_imports_prefixed() {
     addTestFile('''
       import 'dart:html' as foo;
       main() {^}
     ''');
     return getSuggestions().then((_) {
       expect(replacementOffset, equals(completionOffset));
       expect(replacementLength, equals(0));
       assertHasResult(CompletionSuggestionKind.INVOCATION, 'Object');
-      assertHasResult(CompletionSuggestionKind.INVOCATION, 'foo');
+      assertHasResult(CompletionSuggestionKind.IDENTIFIER, 'foo');
       assertNoResult('HtmlElement');
       assertNoResult('test');
     });
   }
 
   test_imports_prefixed2() {
     addTestFile('''
       import 'dart:html' as foo;
       main() {foo.^}
     ''');
@@ skipping 391 matching lines @@
 }
     ''');
     await waitForTasksFinished();
     Request request =
         new CompletionGetSuggestionsParams(testFile, 0).toRequest('0');
     Response response = handler.handleRequest(request);
     expect(response.error, isNotNull);
     expect(response.error.code, RequestErrorCode.NO_INDEX_GENERATED);
   }
 }