Revert "Remove Expect from core library."

tests/compiler/dart2js/string_escapes_test.dart

 // Copyright (c) 2011, 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';
 
 // Test that the compiler handles string literals containing line terminators.
 
 String compileExpression(String expression) {
   var source = "foo() { return $expression; }";
   return compile(source, entry: "foo");
 }
 
 main() {
   String generated = compileExpression("''' \n\r\u2028\u2029'''");
   Expect.isTrue(generated.contains(r"\n\r\u2028\u2029"));
 
   generated = compileExpression("r''' \n\r\u2028\u2029'''");
   Expect.isTrue(generated.contains(r"\n\r\u2028\u2029"));
 
   generated = compileExpression("'\u2028\u2029'");
   Expect.isTrue(generated.contains(r"\u2028\u2029"));
 }