Revert "Remove Expect from core library."

tests/language/cascade_precedence_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.
 
-import "package:expect/expect.dart";
-
 
 class A {
   int value;
   A(this.value);
   void set(int value) { this.value = value; }
   int get() => value;
   int operator[](int index) => value + index;
   void operator[]=(int index, int newValue) { value += -index + newValue; }
   void test(int expected) {
     Expect.equals(expected, value);
@@ skipping 113 matching lines @@
   // Should parse as:
   // box = (box..x = (a.value == 21 ? b : c)..x.test(117));
   box = box..x = a.value == 21 ? b : c..x.test(117);
   Expect.equals(originalBox, box);
   Expect.equals(box.value, b);
 
   // New cascades are allowed inside an expressionWithoutCascade if properly
   // delimited.
   box..x = (a..set(42)..test(42))..x.test(42);
 }