Add SExpression test and share tests between dart2dart and analyzer2dart.

tests/compiler/dart2js/backend_dart/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.
 
-import '../compiler_helper.dart';
+library dart_backend.test_helper;
+
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/implementation/dart2jslib.dart';
-import 'package:compiler/implementation/dart_backend/dart_backend.dart';
-import 'package:compiler/implementation/dart_backend/backend_ast_to_frontend_ast.dart'
-    show INSERT_NEW_BACKEND_COMMENT;
+import '../../../../pkg/analyzer2dart/test/test_helper.dart';
+import '../compiler_helper.dart';
 
-const String TestStaticField = """
-class Foo {
-  static int x = 1;
-}
-main() {
-  print(Foo.x);
-}
-""";
-
-Future<String> compile(String code) {
+/// Compiles the given dart code (which must include a 'main' function) and
+/// returns the compiler.
+Future<Compiler> compilerFor(String code) {
   MockCompiler compiler = new MockCompiler.internal(
       emitJavaScript: false,
       enableMinification: false);
+  compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler.init().then((_) {
     compiler.parseScript(code);
+
     Element element = compiler.mainApp.find('main');
     if (element == null) return null;
+
     compiler.mainFunction = element;
     compiler.phase = Compiler.PHASE_RESOLVING;
     compiler.backend.enqueueHelpers(compiler.enqueuer.resolution,
                                     compiler.globalDependencies);
     compiler.processQueue(compiler.enqueuer.resolution, element);
     compiler.world.populate();
     compiler.backend.onResolutionComplete();
+
     compiler.irBuilder.buildNodes(useNewBackend: true);
-    compiler.phase = Compiler.PHASE_COMPILING;
-    DartBackend backend = compiler.backend;
-    backend.assembleProgram();
-    String generated = compiler.assembledCode;
-    return generated;
+
+    return compiler;
   });
 }
 
-Future test(String code, String feature) {
-  return compile(code).then((output) {
-    if (output == null || !output.contains('main() /* new backend */')) {
-      throw 'New backend appears to be deactivated for $feature.\n'
-            'Output was: $output';
-    }
-  });
+/// Test group using async_helper.
+asyncTester(Group group, RunTest runTest) {
+  asyncTest(() => Future.forEach(group.results, runTest));
 }
-
-main() {
-  INSERT_NEW_BACKEND_COMMENT = true;
-
-  asyncTest(() => Future.forEach([
-    () => test(TestStaticField, 'static fields'),
-  ], (f) => f()));
-}