Revert "Remove Expect from core library."

tests/language/compile_time_constant_o_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.
 // Test compile-time constants with string-interpolation.
 
-import "package:expect/expect.dart";
-
 const str = "foo";
 const m1 = const { "foo": 499 };
 const m2 = const { "$str": 499 };
 const m3 = const { "$str": 42, "foo": 499 };
 const m4 = const { "foo": 42, "$str": 499 };
 const m5 = const { "f" "o" "o": 499 };
 
 const mm1 = const { "afoo#foo": 499 };
 const mm2 = const { "a$str#$str": 499 };
 const mm3 = const { "a" "$str" "#" "foo": 499 };
 const mm4 = const { "a$str" "#$str": 499 };
 
 main() {
   Expect.equals(1, m1.length);
   Expect.equals(499, m1["foo"]);
   Expect.identical(m1, m2);
   Expect.identical(m1, m3);
   Expect.identical(m1, m4);
   Expect.identical(m1, m5);
 
   Expect.equals(1, mm1.length);
   Expect.equals(499, mm1["afoo#foo"]);
   Expect.identical(mm1, mm2);
   Expect.identical(mm1, mm3);
   Expect.identical(mm1, mm4);
 }