Remove a couple of unnecessary is checks

pkg/analyzer/lib/src/context/context.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.context;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 791 matching lines @@
       return true;
     }
     return source.exists();
   }
 
   @override
   CacheEntry getCacheEntry(AnalysisTarget target) {
     CacheEntry entry = _cache.get(target);
     if (entry == null) {
       entry = new CacheEntry(target);
+      ImplicitAnalysisEvent event = null;
       if (target is Source) {
         entry.modificationTime = getModificationStamp(target);
+        event = new ImplicitAnalysisEvent(target, true);
       }
       _cache.put(entry);
-      if (target is Source) {
-        _implicitAnalysisEventsController
-            .add(new ImplicitAnalysisEvent(target, true));
+      if (event != null) {
+        _implicitAnalysisEventsController.add(event);
       }
     }
     return entry;
   }
 
   @override
   CompilationUnitElement getCompilationUnitElement(
       Source unitSource, Source librarySource) {
     AnalysisTarget target = new LibrarySpecificUnit(librarySource, unitSource);
     return getResult(target, COMPILATION_UNIT_ELEMENT);
@@ skipping 1366 matching lines @@
     }
     DartSdk sdk = factory.dartSdk;
     if (sdk == null) {
       throw new IllegalArgumentException(
           "The source factory for an SDK analysis context must have a DartUriResolver");
     }
     return new AnalysisCache(
         <CachePartition>[AnalysisEngine.instance.partitionManager.forSdk(sdk)]);
   }
 }