Add SExpression test and share tests between dart2dart and 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<TestSpec> results;
+
+  const Group(this.name, this.results);
+}
+
+/// A input/output pair that defines the expected [output] of when processing
+/// the [input].
+class TestSpec {
+  final String input;
+  final String output;
+
+  const TestSpec(this.input, this.output);
+}
+
+typedef TestGroup(Group group, RunTest check);
+typedef RunTest(TestSpec result);
+
+/// Test [data] using [testGroup] and [check].
+void performTests(List<Group> data, TestGroup testGroup, RunTest runTest) {
+  for (Group group in data) {
+    testGroup(group, runTest);
+  }
+}
+
+/// Test group using unittest.
+unittester(Group group, RunTest runTest) {
+  test(group.name, () {
+    for (TestSpec result in group.results) {
+      runTest(result);
+    }
+  });
+}