Make Source.getDirectory synchronous.

test/version_solver_test.dart

Index: test/version_solver_test.dart
diff --git a/test/version_solver_test.dart b/test/version_solver_test.dart
index 35fbb9b0683a0153564a637651cbc756899aec0e..070d82897ad9386318d25938e94a3231568b00e1 100644
--- a/test/version_solver_test.dart
+++ b/test/version_solver_test.dart
@@ -1392,9 +1392,7 @@ class MockSource extends CachedSource {
   bool descriptionsEqual(description1, description2) =>
       description1 == description2;
 
-  Future<String> getDirectory(PackageId id) {
-    return new Future.value('${id.name}-${id.version}');
-  }
+  String getDirectory(PackageId id) => '${id.name}-${id.version}';
 
   Future<List<Pubspec>> getVersions(String name, String description) {
     return new Future.sync(() {