Use the cached pubspec if possible for describing hosted packages.

utils/tests/pub/update/pub_update_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_tests;
 
 import 'dart:io';
 
 import 'package:scheduled_test/scheduled_test.dart';
 
@@ skipping 66 matching lines @@
 
     d.dir(appPath, [
       d.pubspec({"name": "myapp", "dependencies": {"foo": {"path": "../foo"}}})
     ]).create();
 
     schedulePub(args: ['update'],
         output: new RegExp(r"Dependencies updated!$"));
 
     d.packagesDir({"foo": null}).validate();
   });
+
+  integration('does not request a pubspec for a cached package', () {
+    servePackages([packageMap("foo", "1.2.3")]);
+
+    d.appDir([dependencyMap("foo", "1.2.3")]).create();
+
+    // Run install once so it gets cached.
+    schedulePub(args: ['install'],
+        output: new RegExp("Dependencies installed!\$"));
+
+    // Clear the cache. We don't care about anything that was served during
+    // the initial install.
+    getRequestedPaths();
+
+    d.cacheDir({"foo": "1.2.3"}).validate();
+    d.packagesDir({"foo": "1.2.3"}).validate();
+
+    schedulePub(args: ['update'],
+        output: new RegExp("Dependencies updated!\$"));
+
+    // The update should not have requested the pubspec since it's installed
+    // locally already.
+    getRequestedPaths().then((paths) {
+      expect(paths, isNot(contains("packages/foo/versions/1.2.3.yaml")));
+    });
+  });
 }