Revert "Remove Expect from core library."

tests/corelib/string_fromcharcode_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";
-
 main() {
   Expect.equals("A", new String.fromCharCode(65));
   Expect.equals("B", new String.fromCharCode(66));
   var gClef = new String.fromCharCode(0x1D11E);
   Expect.equals(2, gClef.length);
   Expect.equals(0xD834, gClef.codeUnitAt(0));
   Expect.equals(0xDD1E, gClef.codeUnitAt(1));
 
   // Unmatched surrogates.
   var unmatched = new String.fromCharCode(0xD800);
   Expect.equals(1, unmatched.length);
   Expect.equals(0xD800, unmatched.codeUnitAt(0));
   unmatched = new String.fromCharCode(0xDC00);
   Expect.equals(1, unmatched.length);
   Expect.equals(0xDC00, unmatched.codeUnitAt(0));
 
   Expect.throws(() => new String.fromCharCode(-1),
                 (e) => e is ArgumentError);
 
   // Invalid code point.
   Expect.throws(() => new String.fromCharCode(0x110000),
                 (e) => e is ArgumentError);
 
   Expect.throws(() => new String.fromCharCode(0x110001),
                 (e) => e is ArgumentError);
 }