Support default constructors in analyzer2dart.

pkg/analyzer2dart/test/test_helper.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// Helpers for defining input/output based unittests through (constant) data.
 
 import 'package:unittest/unittest.dart';
 
 /// A unittest group with a name and a list of input/output results.
 class Group {
   final String name;
   final List<TestSpecBase> results;
 
   const Group(this.name, this.results);
 }
 
 /// A [input] for which a certain processing result is expected.
 class TestSpecBase {
   final String input;
 
   const TestSpecBase(this.input);
 }
 
 typedef TestGroup(Group group, RunTest check);
 typedef RunTest(TestSpecBase result);
 
 /// Test [data] using [testGroup] and [check].
-void performTests(List<Group> data, TestGroup testGroup, RunTest runTest) {
+void performTests(List<Group> data,
+                  TestGroup testGroup,
+                  RunTest runTest,
+                  List<String> groupsToRun) {
   for (Group group in data) {
+    if (groupsToRun.isNotEmpty &&
+        !groupsToRun.contains(group.name)) {
+      // Skip this group.
+      continue;
+    }
     testGroup(group, runTest);
   }
 }
 
 /// Test group using unittest.
 unittester(Group group, RunTest runTest) {
   test(group.name, () {
     for (TestSpecBase result in group.results) {
       runTest(result);
     }
   });
 }