Split installing a package locally from symlinking to it.

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

Index: sdk/lib/_internal/pub/lib/src/source.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source.dart b/sdk/lib/_internal/pub/lib/src/source.dart
index 8cfec0ff8d7bf4caa8aa4bb7660e95cfe02b0670..823b86023d7df38b36bf2a623661670137d8da3f 100644
--- a/sdk/lib/_internal/pub/lib/src/source.dart
+++ b/sdk/lib/_internal/pub/lib/src/source.dart
@@ -94,11 +94,17 @@ abstract class Source {
   /// external code should not call this. Instead, call [describe].
   Future<Pubspec> doDescribe(PackageId id);
 
-  /// Gets the package identified by [id] and places it at [path].
+  /// Ensures that the package identified by [id] is present on the local file
+  /// system.
   ///
-  /// Returns a [Future] that completes when the operation finishes. [path] is
-  /// guaranteed not to exist, and its parent directory is guaranteed to exist.
-  Future get(PackageId id, String path);
+  /// For cached sources, this ensures the package is in the system cache. (If
+  /// already cached, it does nothing.) For uncached sources, it does nothing
+  /// since the package is already local.
+  Future ensureLocal(PackageId id);
+
+  /// Ensures [id] is available locally and creates a symlink at [symlink]
+  /// pointing it.
+  Future get(PackageId id, String symlink);
 
   /// Returns the directory where this package can (or could) be found locally.
   ///