Fix a couple of warnings.

sdk/lib/_internal/pub/test/version_solver_test.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 pub_update_test;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:unittest/unittest.dart';
@@ skipping 807 matching lines @@
         var name = parts[0];
         var version = parts[1];
 
         var package = mockPackage(name, version, dependencies);
         if (name == 'myapp') {
           // Don't add the root package to the server, so we can verify that Pub
           // doesn't try to look up information about the local package on the
           // remote server.
           root = package;
         } else {
-          cache.sources[source].addPackage(name, package);
+          (cache.sources[source] as MockSource).addPackage(name, package);
         }
       });
     });
 
     // Clean up the expectation.
     if (result != null) {
       var newResult = {};
       result.forEach((name, version) {
         parseSource(name, (isDev, name, source) {
           version = new Version.parse(version);
@@ skipping 327 matching lines @@
   var source = "mock1";
   var match = new RegExp(r"(.*) from (.*)").firstMatch(description);
   if (match != null) {
     name = match[1];
     source = match[2];
     if (source == "root") source = null;
   }
 
   callback(isDev, name, source);
 }