Don't schedule SOURCE_KIND computation when it is know already.

pkg/analyzer/lib/src/task/dart_work_manager.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 analyzer.src.task.dart_work_manager;
 
 import 'dart:collection';
 
 import 'package:analyzer/src/context/cache.dart';
 import 'package:analyzer/src/generated/engine.dart'
@@ skipping 99 matching lines @@
    * to be computed with priority.
    */
   void addPriorityResult(AnalysisTarget target, ResultDescriptor result) {
     priorityResultQueue.add(new TargetedResult(target, result));
   }
 
   @override
   void applyChange(List<Source> addedSources, List<Source> changedSources,
       List<Source> removedSources) {
     addedSources = addedSources.where(_isDartSource).toList();
-    changedSources = changedSources.where(_isDartSource).toList();
+    changedSources = changedSources
+        .where(_isDartSource)
+        .where((source) => _needsComputing(source, SOURCE_KIND))
+        .toList();
     removedSources = removedSources.where(_isDartSource).toList();
     // unknown queue
     unknownSourceQueue.addAll(addedSources);
     unknownSourceQueue.addAll(changedSources);
     unknownSourceQueue.removeAll(removedSources);
     // library queue
     librarySourceQueue.removeAll(changedSources);
     librarySourceQueue.removeAll(removedSources);
     // parts in libraries
     for (Source changedSource in changedSources) {
@@ skipping 339 matching lines @@
     }
   }
 
   bool _shouldErrorsBeComputed(Source source) =>
       context.shouldErrorsBeAnalyzed(source);
 
   static bool _isDartSource(AnalysisTarget target) {
     return target is Source && AnalysisEngine.isDartFileName(target.fullName);
   }
 }