enhance code completion tracking and measurement

pkg/analysis_server/lib/src/services/completion/dart_completion_manager.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 services.completion.dart;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/protocol.dart';
 import 'package:analysis_server/src/services/completion/arglist_computer.dart';
@@ skipping 57 matching lines @@
   factory DartCompletionManager.create(AnalysisContext context,
       SearchEngine searchEngine, Source source) {
     return new DartCompletionManager(
         context,
         searchEngine,
         source,
         new DartCompletionCache(context, source));
   }
 
   @override
-  void computeCache() {
-    waitForAnalysis().then((CompilationUnit unit) {
+  Future<bool> computeCache() {
+    return waitForAnalysis().then((CompilationUnit unit) {
       if (unit != null && !cache.isImportInfoCached(unit)) {
-        cache.computeImportInfo(unit, searchEngine);
+        return cache.computeImportInfo(unit, searchEngine);
+      } else {
+        return new Future.value(false);
       }
     });
   }
 
   /**
    * Compute suggestions based upon cached information only
    * then send an initial response to the client.
    * Return a list of computers for which [computeFull] should be called
    */
   List<DartCompletionComputer> computeFast(DartCompletionRequest request) {
@@ skipping 187 matching lines @@
   _ReplacementOffsetBuilder(this.request) {
     request.replacementOffset = request.offset;
     request.replacementLength = 0;
   }
 
   visitSimpleIdentifier(SimpleIdentifier node) {
     request.replacementOffset = node.offset;
     request.replacementLength = node.length;
   }
 }