Revert "Remove Expect from core library."

tests/language/type_checks_in_factory_method_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.
 // VMOptions=--enable_checked_mode
 // Tests the type checking when passing code into closure from inside a factory method
 
-import "package:expect/expect.dart";
-
 abstract class Foo<T> {
   factory Foo.from() = Bar<T>.from;
 }
 
 class Bar<T> implements Foo<T> {
   Bar() {}
 
   factory Bar.from() {
     var func = (T arg) {
       T foo = arg;
@@ skipping 13 matching lines @@
 
   bool gotError = false;
 
   try {
     Foo<int> value2 = new Foo<int>.from();
   } on TypeError catch (e) {
     gotError = true;
   }
   Expect.equals(true, gotError);
 }