Revert "Remove Expect from core library."

tests/compiler/dart2js/tree_shaking_test.dart

 // Copyright (c) 2012, 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';
 
 const String TEST = r"""
 class A {
   foo() => bar();
   bar() => 42;
 }
 class B extends A {
   bar() => 54;
 }
 class C implements A {
   bar() => 68;
 }
 main() {
   new A();
   new B();
   new C();
   new A().foo();
 }
 """;
 
 void main() {
   String generated = compileAll(TEST);
   Expect.isTrue(generated.contains('return 42'));
   Expect.isTrue(generated.contains('return 54'));
   Expect.isFalse(generated.contains('return 68'));
 }