Fix recently introduced strong mode errors

pkg/analyzer/lib/src/task/dart.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;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 1578 matching lines @@
     }
     //
     // Try to get the existing LibraryElement.
     //
     LibraryElementImpl libraryElement;
     {
       InternalAnalysisContext internalContext =
           context as InternalAnalysisContext;
       AnalysisCache analysisCache = internalContext.analysisCache;
       CacheEntry cacheEntry = internalContext.getCacheEntry(target);
-      libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1);
+      libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1) as LibraryElementImpl;
       if (libraryElement == null &&
           internalContext.aboutToComputeResult(cacheEntry, LIBRARY_ELEMENT1)) {
-        libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1);
+        libraryElement = analysisCache.getValue(target, LIBRARY_ELEMENT1) as LibraryElementImpl;
       }
     }
     //
     // Create a new LibraryElement.
     //
     if (libraryElement == null) {
       libraryElement =
           new LibraryElementImpl.forNode(owningContext, libraryNameNode);
       libraryElement.synthetic = modificationTime < 0;
       libraryElement.definingCompilationUnit = definingCompilationUnitElement;
@@ skipping 4740 matching lines @@
 
   @override
   bool moveNext() {
     if (_newSources.isEmpty) {
       return false;
     }
     currentTarget = _newSources.removeLast();
     return true;
   }
 }