Revert "Remove Expect from core library."

tests/language/function_type_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 a function type test that cannot be eliminated at compile time.
 
-import "package:expect/expect.dart";
-
 isCheckedMode() {
   try {
     var i = 1;
     String s = i;
     return false;
   } catch (e) {
     return true;
   }
 }
 
@@ skipping 10 matching lines @@
     // Static result type of f(), i.e. FList, is a subtype of FListInt.
     // However, run time type of returned function is not a subtype of FListInt.
     // Run time type check should not be eliminated.
     FListInt fli = f();
   } on TypeError catch (error) {
     got_type_error = true;
   }
   // Type error expected in checked mode only.
   Expect.isTrue(got_type_error == isCheckedMode());
 }