Remove deprecated string features.

tests/language/instance_inline_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 inlining of assignments in parameter passing. If [StringScanner.charAt]
 // is inlined, the argument expresion [: ++byteOffset :] should not be
 // duplicated.
 
 class StringScanner {
   final String string;
   int byteOffset = -1;
 
   StringScanner(this.string);
 
   int nextByte() => charAt(++byteOffset);
 
   int charAt(index)
-      => (string.length > index) ? string.charCodeAt(index) : -1;
+      => (string.length > index) ? string.codeUnitAt(index) : -1;
 }
 
 void main() {
   var scanner = new StringScanner('az9');
   Expect.equals(0x61, scanner.nextByte()); // Expect a.
   Expect.equals(0x7A, scanner.nextByte()); // Expect z.
   Expect.equals(0x39, scanner.nextByte()); // Expect 9.
   Expect.equals(-1, scanner.nextByte());
-}
+}