Revert "Remove Expect from core library."

tests/corelib/compare_to2_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 for testing Math.min and Math.max.
 
-import "package:expect/expect.dart";
-
 negate(x) => -x;
 
 main() {
   // Test matrix:
   var minNonZero = 5e-324;
   var maxDenormal = 2.225073858507201e-308;
   var minNormal = 2.2250738585072014e-308;
   var maxFraction = 0.9999999999999999;
   var minAbove1 = 1.0000000000000002;
   var maxNonInt = 4503599627370495.5;
@@ skipping 65 matching lines @@
     for (int j = 0; j < matrix.length; j++) {
       var left = matrix[i];
       var right = matrix[j];
       if (left is List) {
         check(left, left, 0);
       }
       check(left, right, i == j ? 0 : (i < j ? -1 : 1));
     }
   }
 }