remove CompletionCache and obsolete tests, and cleanup imports

pkg/analysis_server/test/services/completion/dart/override_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.override;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     hide Element, ElementKind;
 import 'package:analysis_server/src/provisional/completion/dart/completion_dart.dart';
 import 'package:analysis_server/src/services/completion/dart/override_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 'completion_contributor_util.dart';
 
 main() {
   // Revisit this contributor and these tests
   // once DartChangeBuilder API has solidified.
   // initializeTestEnvironment();
   // defineReflectiveTests(InheritedContributorTest);
@@ skipping 89 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;
   }
 }