enhance code completion tracking and measurement

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/src/analysis_server.dart';
 import 'package:analysis_server/src/channel/channel.dart';
@@ skipping 426 matching lines @@
 class MockCompletionManager implements CompletionManager {
   final AnalysisContext context;
   final Source source;
   final SearchEngine searchEngine;
   StreamController<CompletionResult> controller;
   int computeCallCount = 0;
 
   MockCompletionManager(this.context, this.source, this.searchEngine);
 
   @override
-  void computeCache() {
-    // ignored
+  Future<bool> computeCache() {
+    return new Future.value(true);
   }
 
   @override
   void computeSuggestions(CompletionRequest request) {
     ++computeCallCount;
     CompletionResult result = new CompletionResult(0, 0, [], true);
     controller.add(result);
   }
 
   @override
@@ skipping 99 matching lines @@
 
   void contextsChangedRaw(ContextsChangedEvent newEvent) {
     super.contextsChanged(newEvent);
   }
 
   CompletionManager createCompletionManager(AnalysisContext context,
       Source source, SearchEngine searchEngine) {
     return new MockCompletionManager(mockContext, source, searchEngine);
   }
 }