toRadixString returns lower case characters.

runtime/lib/integers.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.
 
 // TODO(srdjan): fix limitations.
 // - shift amount must be a Smi.
 class IntegerImplementation {
   factory IntegerImplementation._uninstantiable() {
     throw const UnsupportedOperationException(
         "IntegerImplementation can only be allocated by the VM");
@@ skipping 119 matching lines @@
     return this.toDouble().toStringAsFixed(fractionDigits);
   }
   String toStringAsExponential(int fractionDigits) {
     return this.toDouble().toStringAsExponential(fractionDigits);
   }
   String toStringAsPrecision(int precision) {
     return this.toDouble().toStringAsPrecision(precision);
   }
   String toRadixString(int radix) {
     final table = const ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9",
-                         "A", "B", "C", "D", "E", "F"];
+                         "a", "b", "c", "d", "e", "f"];
     if ((radix <= 1) || (radix > 16)) {
       throw "Bad radix: $radix";
     }
     final bool isNegative = this < 0;
     var value = isNegative ? -this : this;
     List temp = new List();
     while (value > 0) {
       var digit = value % radix;
       value ~/= radix;
       temp.add(digit);
@@ skipping 68 matching lines @@
     }
   }
   int shlFromInt(int other) {
     throw const OutOfMemoryException();
   }
 
   int pow(int exponent) {
     throw "Bigint.pow not implemented";
   }
 }