Remove unnecessary casts and general code clean-up

pkg/analyzer/lib/src/context/cache.dart

Index: pkg/analyzer/lib/src/context/cache.dart
diff --git a/pkg/analyzer/lib/src/context/cache.dart b/pkg/analyzer/lib/src/context/cache.dart
index 8be0a41b658a3b034501ba2ec492024fec44c3a4..0dcac1de80174d7b95ed67476e8e449575b6a14c 100644
--- a/pkg/analyzer/lib/src/context/cache.dart
+++ b/pkg/analyzer/lib/src/context/cache.dart
@@ -315,8 +315,15 @@ class CacheEntry {
       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);
     }
   }