Split installing a package locally from symlinking to it.

sdk/lib/_internal/pub/test/pubspec_test.dart

Index: sdk/lib/_internal/pub/test/pubspec_test.dart
diff --git a/sdk/lib/_internal/pub/test/pubspec_test.dart b/sdk/lib/_internal/pub/test/pubspec_test.dart
index b4361a3efbe532d8a41b1c1690597f9ba0948c50..29c133ac4d823c9888a98acf08d2f2e38b7fe9f1 100644
--- a/sdk/lib/_internal/pub/test/pubspec_test.dart
+++ b/sdk/lib/_internal/pub/test/pubspec_test.dart
@@ -21,7 +21,10 @@ class MockSource extends Source {
   Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
       "Cannot describe mock packages.");
 
-  Future<bool> get(PackageId id, String path) => throw new UnsupportedError(
+  Future ensureLocal(PackageId id) => throw new UnsupportedError(
+      "Cannot get a mock package.");
+
+  Future get(PackageId id, String symlink) => throw new UnsupportedError(
       "Cannot get a mock package.");
 
   Future<String> getDirectory(PackageId id) => throw new UnsupportedError(