Use memory compiler in compiler_helper.dart.

tests/compiler/dart2js/arithmetic_simplification_test.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.
 // Test constant folding on numbers.
 
 import 'dart:async';
 import 'package:async_helper/async_helper.dart';
 import 'compiler_helper.dart';
 
 const String INT_PLUS_ZERO = """
@@ skipping 63 matching lines @@
 
 main() {
   var plusZero = new RegExp(r"\+ 0");
   var zeroPlus = new RegExp(r"0 \+");
   var timesOne = new RegExp(r"\* 1");
   var oneTimes = new RegExp(r"1 \*");
 
   asyncTest(() => Future.wait([
     compileAndDoNotMatch(INT_PLUS_ZERO, 'main', plusZero),
     compileAndDoNotMatch(ZERO_PLUS_INT, 'main', zeroPlus),
-    compileAndMatch(NUM_PLUS_ZERO, 'main', plusZero),
+    // TODO(johnniwinther): Find out why this doesn't work without [useMock].
+    compileAndMatch(NUM_PLUS_ZERO, 'main', plusZero, useMock: true),
     compileAndMatch(ZERO_PLUS_NUM, 'main', zeroPlus),
     compileAndDoNotMatch(INT_TIMES_ONE, 'main', timesOne),
     compileAndDoNotMatch(ONE_TIMES_INT, 'main', oneTimes),
     compileAndDoNotMatch(NUM_TIMES_ONE, 'main', timesOne),
     compileAndDoNotMatch(ONE_TIMES_NUM, 'main', oneTimes),
   ]));
 }