Refactor Future constructors.

utils/tests/pub/version_solver_test.dart

Index: utils/tests/pub/version_solver_test.dart
diff --git a/utils/tests/pub/version_solver_test.dart b/utils/tests/pub/version_solver_test.dart
index 94a7c58a4c76c57785887945e003e3b89682eab2..845f597ad0b20293776e04874179014d275df37a 100644
--- a/utils/tests/pub/version_solver_test.dart
+++ b/utils/tests/pub/version_solver_test.dart
@@ -481,11 +481,11 @@ class MockSource extends Source {
       : _packages = <String, Map<Version, Package>>{};
 
   Future<List<Version>> getVersions(String name, String description) {
-    return new Future.of(() => _packages[description].keys.toList());
+    return new Future.sync(() => _packages[description].keys.toList());
   }
 
   Future<Pubspec> describe(PackageId id) {
-    return new Future.of(() => _packages[id.name][id.version].pubspec);
+    return new Future.sync(() => _packages[id.name][id.version].pubspec);
   }
 
   Future<bool> install(PackageId id, String path) {