Revert "Remove Expect from core library."

tests/language/partial_min_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";
-
 // This test triggered an NPE in dart2js.
 // The bug needed:
 // - double usage of the same variable in one expression ("b != b").
 // - replacement of "b" with "typechecked b".
 // - ...
 // The example below is minimal (as far as I was able to do).
 
 num foo(num a, num b) {
   if (a > b) return b;
   if (b is double) {
     if (true) {
       if (true) {
         return (a + b) * a * b;
       }
     }
     // Check for NaN and b == -0.0.
     if (a == 0 && b == 0 || b != b) return b;
   }
 }
 
 main() {
   Expect.equals(1, foo(2, 1));
 }