Split installing a package locally from symlinking to it.

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

Index: sdk/lib/_internal/pub/lib/src/source/cached.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/cached.dart b/sdk/lib/_internal/pub/lib/src/source/cached.dart
index 4fada96e6bb3920f6beaa30ecab62da57f2a87a8..aaafd3190a1122eff406c2a88a3081d8a67ff0a1 100644
--- a/sdk/lib/_internal/pub/lib/src/source/cached.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/cached.dart
@@ -47,9 +47,14 @@ abstract class CachedSource extends Source {
   /// the system cache.
   Future<Pubspec> describeUncached(PackageId id);
 
-  Future get(PackageId id, String packageDir) {
-    return downloadToSystemCache(id).then(
-        (pkg) => createPackageSymlink(id.name, pkg.dir, packageDir));
+  Future ensureLocal(PackageId id) {
+    return downloadToSystemCache(id);
+  }
+
+  Future get(PackageId id, String symlink) {
+    return downloadToSystemCache(id).then((pkg) {
+      createPackageSymlink(id.name, pkg.dir, symlink);
+    });
   }
 
   /// Determines if the package with [id] is already downloaded to the system