Add SExpression test and share tests between dart2dart and analyzer2dart.

pkg/analyzer2dart/test/output_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.
+
+/// Helper classes for testing compiler output.
+library test.output_helper;
+
+import 'dart:async';
+
+
+class CollectingOutputProvider {
+  StringBufferSink output;
+
+  EventSink<String> call(String name, String extension) {
+    return output = new StringBufferSink();
+  }
+}
+
+class StringBufferSink implements EventSink<String> {
+  StringBuffer sb = new StringBuffer();
+
+  void add(String text) {
+    sb.write(text);
+  }
+
+  void addError(errorEvent, [StackTrace stackTrace]) {}
+
+  void close() {}
+
+  String get text => sb.toString();
+}
+