Revert "Remove Expect from core library."

tests/language/range_analysis_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.
 // Dart test program for constructors and initializers.
 
-import "package:expect/expect.dart";
-
 // Check that range analysis does not enter infinite loop trying to propagate
 // ranges through dependant phis.
 bar() {
   var sum = 0;
   for (var i = 0; i < 10; i++) {
     for (var j = i - 1; j >= 0; j--) {
       for (var k = j; k < i; k++) {
         sum += (i + j + k);
       }
     }
@@ skipping 18 matching lines @@
     Expect.isTrue(i > 0);
     x = i;
   }
   return x;
 }
 
 main() {
   test1();
   test2();
 }