Remove deprecated string features.

tests/corelib/string_codeunits_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.
 
 main() {
   test(String s) {
     Iterable<int> units = s.codeUnits;
     List<int> expectedUnits = <int>[];
     for (int i = 0; i < s.length; i++) {
       expectedUnits.add(s.codeUnitAt(i));
@@ skipping 13 matching lines @@
 
     // .map
     Expect.listEquals(expectedUnits.map((x) => x.toRadixString(16)).toList(),
                       units.map((x) => x.toRadixString(16)).toList());
   }
 
   test("abc");
   test("\x00\u0000\u{000000}");
   test("\u{ffff}\u{10000}\u{10ffff}");
   String string = new String.fromCharCodes(
-      [0xdc00, 0xd800, 61, 0xd800, 0xdc00, 62, 0xdc00, 0xd800]);
+      [0xdc00, 0xd800, 61, 0xd9ab, 0xd9ab, 0xddef, 0xddef, 62, 0xdc00, 0xd800]);
+  test(string);
+  string = "\x00\x7f\xff\u0100\ufeff\uffef\uffff"
+           "\u{10000}\u{12345}\u{1d800}\u{1dc00}\u{1ffef}\u{1ffff}";
   test(string);
 
-  // Setting position in the middle of a surrogate pair is not allowed.
-  var r = new CodeUnits("\u{10000}");
+  // Reading each unit of a surrogate pair works.
+  var r = "\u{10000}".codeUnits;
   var it = r.iterator;
   Expect.isTrue(it.moveNext());
   Expect.equals(0xD800, it.current);
   Expect.isTrue(it.moveNext());
   Expect.equals(0xDC00, it.current);
 }