Remove unnecessary casts and general code clean-up

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 356 matching lines @@
       }
     }
     return sources;
   }
 
   @override
   List<Source> get launchableServerLibrarySources {
     List<Source> sources = <Source>[];
     for (Source source in _cache.sources) {
       CacheEntry entry = _cache.get(source);
-      if (source is Source &&
-          entry.getValue(SOURCE_KIND) == SourceKind.LIBRARY &&
+      if (entry.getValue(SOURCE_KIND) == SourceKind.LIBRARY &&
           !source.isInSystemLibrary &&
           isServerLibrary(source)) {
         sources.add(source);
       }
     }
     return sources;
   }
 
   @override
   List<Source> get librarySources => _getSources(SourceKind.LIBRARY);
@@ skipping 1807 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)]);
   }
 }