Remove unnecessary casts and general code clean-up

pkg/analyzer/lib/src/context/cache.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.context.cache;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/src/dart/element/element.dart'
@@ skipping 297 matching lines @@
    */
   int _flags = 0;
 
   /**
    * A table mapping result descriptors to the cached values of those results.
    */
   Map<ResultDescriptor, ResultData> _resultMap =
       new HashMap<ResultDescriptor, ResultData>();
 
   CacheEntry(this.target) {
+    _markAsCacheKey(target);
+  }
+
+  /**
+   * If the given `target` is an element, mark it as being a cache key.
+   */
+  void _markAsCacheKey(AnalysisTarget target) {
     if (target is ElementImpl) {
-      (target as ElementImpl).setModifier(Modifier.CACHE_KEY, true);
+      target.setModifier(Modifier.CACHE_KEY, true);
     }
   }
 
   /**
    * The exception that caused one or more values to have a state of
    * [CacheState.ERROR].
    */
   CaughtException get exception => _exception;
 
   /**
@@ skipping 941 matching lines @@
   void resultAccessed(TargetedResult result) {}
 
   @override
   List<TargetedResult> resultStored(TargetedResult newResult, newValue) {
     return TargetedResult.EMPTY_LIST;
   }
 
   @override
   void targetRemoved(AnalysisTarget target) {}
 }