Remove deprecated string features.

pkg/fixnum/lib/src/int64.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.
 
 part of fixnum;
 
 /**
  * An immutable 64-bit signed integer, in the range [-2^63, 2^63 - 1].
  * Arithmetic operations may overflow in order to maintain this range.
  */
@@ skipping 112 matching lines @@
       throw "Bad radix: $radix";
     }
     int64 x = ZERO;
     int i = 0;
     bool negative = false;
     if (s[0] == '-') {
       negative = true;
       i++;
     }
     for (; i < s.length; i++) {
-      int c = s.charCodeAt(i);
+      int c = s.codeUnitAt(i);
       int digit = int32._decodeHex(c);
       if (digit < 0 || digit >= radix) {
         throw new Exception("Non-radix char code: $c");
       }
       x = (x * radix) + digit;
     }
     return negative ? -x : x;
   }
 
   /**
@@ skipping 944 matching lines @@
         }
       }
       return ZERO;
     }
 
     // Generate the quotient using bit-at-a-time long division.
     return _divModHelper(aIsCopy ? a : new int64._copy(a), b, negative,
         aIsNegative, aIsMinValue, computeRemainder);
   }
 }