Revert "Remove Expect from core library."

tests/compiler/dart2js/scanner_offset_length_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 '../../../sdk/lib/_internal/compiler/implementation/scanner/scannerlib.dart';
 import '../../../sdk/lib/_internal/compiler/implementation/scanner/scanner_implementation.dart';
 
 Token scan(String text) =>
     new StringScanner(text, includeComments: true).tokenize();
 
 check(String text) {
   Token token = scan(text);
   while (token.kind != EOF_TOKEN) {
     Expect.equals(token.slowToString().length, token.slowCharCount);
@@ skipping 24 matching lines @@
 main() {
     check('foo'); // identifier
     check('\'\''); // empty string
     check('\'foo\''); // simple string
     check('\'\$foo\''); // interpolation, identifier
     check('\'\${foo}\''); // interpolation, expression
 
     check('//'); // single line comment
     check('/**/'); // multi line comment
 }