Revert "Remove Expect from core library."

tests/language/issue7513_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";
-
 // Regression test for issue 7513.
 
 foo(a, b) {
   b[0] = 0.1;
   return a * b[0];
 }
 
 main() {
   var a = 0.1;
   var b = [0.1];
   for (var i = 0; i < 10000; i++) {
     foo(a, b);
   }
   Expect.approxEquals(0.01, foo(a, b));
 }