Remove deprecated string features.

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