Revert "Remove Expect from core library."

tests/language/branch_canonicalization_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that branch fusion correctly sets branch environment for comparisons
 // that require unboxing and does not fuse branches that can deoptimize.
 
-import "package:expect/expect.dart";
-
 var sideEffect = true;
 
 barDouble(a, b) {
   sideEffect = false;
   final result = (a == b);
   sideEffect = !sideEffect;
   return result;
 }
 fooDouble(a, b) => barDouble(a, b) ? 1 : 0;
 
@@ skipping 36 matching lines @@
     Expect.equals(1, fooPoly(x, x));
     Expect.equals(0, fooPoly(y, x));
   }
   Expect.equals(1, fooDouble(z, z));
   Expect.isTrue(sideEffect);
   Expect.equals(1, fooMint(z, z));
   Expect.isTrue(sideEffect);
   Expect.equals(1, fooPoly(z, z));
   Expect.isTrue(sideEffect);
 }