Fix some warnings in pub and pkg packages.

utils/pub/package.dart

 // Copyright (c) 2012, 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 package;
 
 import 'dart:async';
 
 import 'package:pathos/path.dart' as path;
 
@@ skipping 28 matching lines @@
   List<PackageRef> get dependencies => pubspec.dependencies;
 
   /// Returns the path to the README file at the root of the entrypoint, or null
   /// if no README file is found. If multiple READMEs are found, this uses the
   /// same conventions as pub.dartlang.org for choosing the primary one: the
   /// README with the fewest extensions that is lexically ordered first is
   /// chosen.
   String get readmePath {
     var readmes = listDir(dir).map(path.basename).
         where((entry) => entry.contains(_README_REGEXP));
-    if (readmes.isEmpty) return;
+    if (readmes.isEmpty) return null;
 
     return path.join(dir, readmes.reduce((readme1, readme2) {
       var extensions1 = ".".allMatches(readme1).length;
       var extensions2 = ".".allMatches(readme2).length;
       var comparison = extensions1.compareTo(extensions2);
       if (comparison == 0) comparison = readme1.compareTo(readme2);
       return (comparison <= 0) ? readme1 : readme2;
     }));
   }
 
@@ skipping 145 matching lines @@
 
 class PubspecNameMismatchException implements Exception {
   final String expectedName;
   final String actualName;
 
   PubspecNameMismatchException(this.expectedName, this.actualName);
 
   String toString() => 'The name you specified for your dependency, '
     '"$expectedName", doesn\'t match the name "$actualName" in its pubspec.';
 }