Remove deprecated string features.

tests/language/invocation_mirror_invoke_on2_test.dart

 // Copyright (c) 2013, 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.
 
 class Proxy {
   final proxied;
   Proxy(this.proxied);
   noSuchMethod(mirror) => mirror.invokeOn(proxied);
 }
 
@@ skipping 30 matching lines @@
   Expect.equals(87, list[1]);
 
   Expect.throws(() => proxy.funky(), (e) => e is NoSuchMethodError);
   Expect.throws(() => list.funky(), (e) => e is NoSuchMethodError);
 }
 
 testString() {
   var string = "funky";
   var proxy = new Proxy(string);
 
-  Expect.equals(string.charCodeAt(0), proxy.charCodeAt(0));
+  Expect.equals(string.codeUnitAt(0), proxy.codeUnitAt(0));
   Expect.equals(string.length, proxy.length);
 
   Expect.throws(() => proxy.funky(), (e) => e is NoSuchMethodError);
   Expect.throws(() => string.funky(), (e) => e is NoSuchMethodError);
 }
 
 testInt() {
   var number = 42;
   var proxy = new Proxy(number);
 
   Expect.equals(number + 87, proxy + 87);
   Expect.equals(number.toDouble(), proxy.toDouble());
 
   Expect.throws(() => proxy.funky(), (e) => e is NoSuchMethodError);
   Expect.throws(() => number.funky(), (e) => e is NoSuchMethodError);
 }
 
 testDouble() {
   var number = 42.99;
   var proxy = new Proxy(number);
 
   Expect.equals(number + 87, proxy + 87);
   Expect.equals(number.toInt(), proxy.toInt());
 
   Expect.throws(() => proxy.funky(), (e) => e is NoSuchMethodError);
   Expect.throws(() => number.funky(), (e) => e is NoSuchMethodError);
 }