Specialize _toPow2String for bigint to make it linear instead of quadratic.

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 extends _Num {
   // The Dart class _Bigint extending _IntegerImplementation requires a
   // default constructor.
 
@@ skipping 202 matching lines @@
     return this.toDouble().toStringAsPrecision(precision);
   }
 
   static const _digits = "0123456789abcdefghijklmnopqrstuvwxyz";
 
   String toRadixString(int radix) {
     if (radix is! int || radix < 2 || radix > 36) {
       throw new ArgumentError(radix);
     }
     if (radix & (radix - 1) == 0) {
-      return _toPow2String(this, radix);
+      return _toPow2String(radix);
     }
     if (radix == 10) return this.toString();
     final bool isNegative = this < 0;
     int value = isNegative ? -this : this;
     List temp = new List();
     do {
       int digit = value % radix;
       value ~/= radix;
       temp.add(_digits.codeUnitAt(digit));
     } while (value > 0);
     if (isNegative) temp.add(0x2d);  // '-'.
 
     _OneByteString string = _OneByteString._allocate(temp.length);
     for (int i = 0, j = temp.length; j > 0; i++) {
       string._setAt(i, temp[--j]);
     }
     return string;
   }
 
-  static String _toPow2String(value, radix) {
+  String _toPow2String(int radix) {
+    int value = this;
     if (value == 0) return "0";
     assert(radix & (radix - 1) == 0);
     var negative = value < 0;
     var bitsPerDigit = radix.bitLength - 1;
     var length = 0;
     if (negative) {
       value = -value;
       length = 1;
     }
     // Integer division, rounding up, to find number of _digits.
@@ skipping 248 matching lines @@
   // Shift by mint exceeds range that can be handled by the VM.
   int _shrFromInt(int other) {
     if (other < 0) {
       return -1;
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Mint_shlFromInt";
 }