First batch of changes to make analyzer package strong mode error free

pkg/analyzer/lib/source/sdk_ext.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.source.sdk_ext;
 
 import 'dart:convert';
 import 'dart:core' hide Resource;
 
 import 'package:analyzer/file_system/file_system.dart';
@@ skipping 137 matching lines @@
       return;
     }
     var key = name;
     var value = libDir.canonicalizePath(file);
     _urlMappings[key] = value;
   }
 
   /// Read the contents of [libDir]/[SDK_EXT_NAME] as a string.
   /// Returns null if the file doesn't exist.
   String _readDotSdkExt(Folder libDir) {
-    var file = libDir.getChild(SDK_EXT_NAME);
+    File file = libDir.getChild(SDK_EXT_NAME);
     try {
       return file.readAsStringSync();
     } on FileSystemException {
       // File can't be read.
       return null;
     }
   }
 
   /// Resolve an import of an sdk extension.
   Source _resolveEntry(Uri libraryEntry, Uri importUri) {
     // Library entry.
     JavaFile javaFile = new JavaFile.fromUri(libraryEntry);
     return new FileBasedSource(javaFile, importUri);
   }
 
   /// Resolve a 'part' statement inside an sdk extension.
   Source _resolvePart(Uri libraryEntry, String partPath, Uri importUri) {
     // Library part.
     var directory = pathos.dirname(libraryEntry.path);
     var partUri = new Uri.file(pathos.join(directory, partPath));
     assert(partUri.isAbsolute);
     JavaFile javaFile = new JavaFile.fromUri(partUri);
     return new FileBasedSource(javaFile, importUri);
   }
 }