Revert "Remove Expect from core library."

tests/language/prefix16_test.dart

 // Copyright (c) 2011, 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.
 //
 // Unresolved imported symbols are handled differently in production mode and
 // check modes. In this test, the function myFunc is malformed, because
 // lib12.Library13 is not resolved.
 // In checked mode, the assignment type check throws a run time type error.
 // In production, no assignment type checks are performed.
 
 library Prefix16NegativeTest.dart;
-import "package:expect/expect.dart";
 import "library12.dart" as lib12;
 
 typedef lib12.Library13 myFunc(lib12.Library13 param);
 
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
     return false;
   } catch (e) {
@@ skipping 17 matching lines @@
     try {
       // In production mode, malformed myFunc is mapped to (dynamic) => dynamic.
       Expect.isTrue(((int x) => x) is myFunc);
     } on TypeError catch (error) {
       got_type_error = true;
     }
     // Type error in checked mode only.
     Expect.isTrue(got_type_error == isCheckedMode());
   }
 }