create new Dart specific completion contributor extension point

pkg/analysis_server/test/services/completion/dart/inherited_contributor_test.dart

 // Copyright (c) 2015, 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.services.completion.dart.inherited_computer_test;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     hide Element, ElementKind;
-import 'package:analysis_server/src/provisional/completion/completion_dart.dart';
+import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/services/completion/dart/inherited_contributor.dart';
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart'
     hide DartCompletionContributor;
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 import 'package:unittest/unittest.dart';
 
-// import '../../../utils.dart';
 import 'completion_contributor_util.dart';
 
 main() {
   // Revisit this contributor and these tests
   // once DartChangeBuilder API has solidified.
   // initializeTestEnvironment();
   // defineReflectiveTests(InheritedContributorTest);
 }
 
 @reflectiveTest
@@ skipping 36 matching lines @@
         '/myLib.dart',
         '''
 library myLib;
 part '$testFile'
 part '/otherPart.dart'
 class A {
   A suggested1(int x) => null;
   B suggested2(String y) => null;
 }
 ''');
-  addSource(
-      '/otherPart.dart',
-      '''
+    addSource(
+        '/otherPart.dart',
+        '''
 part of myLib;
 class B extends A {
   B suggested2(String y) => null;
   C suggested3([String z]) => null;
 }
 ''');
     addTestSource(r'''
 part of myLib;
 class C extends B {
   sugg^
@@ skipping 27 matching lines @@
     expect(cs.relevance, equals(DART_RELEVANCE_HIGH));
     expect(cs.importUri, null);
 //    expect(cs.selectionOffset, equals(completion.length));
 //    expect(cs.selectionLength, equals(0));
     expect(cs.isDeprecated, isFalse);
     expect(cs.isPotential, isFalse);
     expect(cs.element, isNotNull);
     return cs;
   }
 }