Use the cached pubspec if possible for describing hosted packages.

utils/tests/pub/update/pub_update_test.dart

Index: utils/tests/pub/update/pub_update_test.dart
diff --git a/utils/tests/pub/update/pub_update_test.dart b/utils/tests/pub/update/pub_update_test.dart
index 8242d887cf39cd795b1b3b4ebd87099f0e8e0d8e..160c6a5db13239bb987cfcf27456590cecc52e2f 100644
--- a/utils/tests/pub/update/pub_update_test.dart
+++ b/utils/tests/pub/update/pub_update_test.dart
@@ -84,4 +84,30 @@ main() {
 
     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")));
+    });
+  });
 }