Split installing a package locally from symlinking to it.

sdk/lib/_internal/pub/lib/src/source/unknown.dart

Index: sdk/lib/_internal/pub/lib/src/source/unknown.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/unknown.dart b/sdk/lib/_internal/pub/lib/src/source/unknown.dart
index 5fcd27389281d395414d9507d6b463c16edbbdc8..da20dc2bc5a8e05576da9202a58de7ba34a15cf2 100644
--- a/sdk/lib/_internal/pub/lib/src/source/unknown.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/unknown.dart
@@ -31,9 +31,12 @@ class UnknownSource extends Source {
   Future<Pubspec> doDescribe(PackageId id) => throw new UnsupportedError(
       "Cannot describe a package from unknown source '$name'.");
 
-  Future<bool> get(PackageId id, String path) => throw new UnsupportedError(
+  Future ensureLocal(PackageId id) => throw new UnsupportedError(
       "Cannot get a package from an unknown source '$name'.");
 
+  Future get(PackageId id, String symlink) => throw new UnsupportedError(
+      "Cannot get an unknown source '$name'.");
+
   /// Returns the directory where this package can be found locally.
   Future<String> getDirectory(PackageId id) => throw new UnsupportedError(
       "Cannot find a package from an unknown source '$name'.");