Revert "Remove Expect from core library."

tests/language/adjacent_const_string_literals_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";
-
 
 class Conster {
   const Conster(this.value);
 
   final value;
 
   toString() {
     return value.toString();
   }
 }
@@ skipping 72 matching lines @@
 a""" "b" "e")).toString());
   Expect.equals("a b e", (const Conster("""
 a""" " b" " e")).toString());
 
   Expect.equals("abe", (const Conster("""
 a""" """
 b""" """
 e""")).toString());
 }