Fix some tests.

tests/compiler/dart2js/interpolation_folding_test.dart

 // Copyright (c) 2013, 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 'package:expect/expect.dart';
 import 'compiler_helper.dart';
 
 // Tests for
 const String TEST_1 = r"""
   foo() {
     int a = 120;
     String b = 'hello';
     return 'u${a}v${b}w';
   }
 """;
@@ skipping 33 matching lines @@
   check(TEST_2, r'"xxxxxyyyyy"');
 
   // 1. No merging of fragments that are multi-use.  Prevents exponential code
   //    and keeps author's manual CSE.
   // 2. Know string values require no stringification.
   check(TEST_3, r'return b + "x" + b');
 
   // Known int value can be formatted directly.
   check(TEST_4, r'return "" + b.length');
 }