Revert modPow introduction since dart2js does not like it.

runtime/lib/bigint.dart

 // Copyright (c) 2014, 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.
 
 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 /*
  * Copyright (c) 2003-2005  Tom Wu
@@ skipping 1310 matching lines @@
     other._toBigint()._divRemTo(this, null, result);
     return result._toValidInt();
   }
   bool _greaterThanFromInteger(int other) {
     return other._toBigint()._compareTo(this) > 0;
   }
   bool _equalToInteger(int other) {
     return other._toBigint()._compareTo(this) == 0;
   }
 
+  // TODO(regis): Make this method private once the plumbing to invoke it from
+  // dart:math is in place. Move the argument checking to dart:math.
   // Return pow(this, e) % m.
   int modPow(int e, int m) {
     if (e is! int) throw new ArgumentError(e);
     if (m is! int) throw new ArgumentError(m);
     int i = e.bitLength;
     if (i <= 0) return 1;
     if ((e is! _Bigint) || m.isEven) {
       _Reduction z = (i < 8 || m.isEven) ? new _Classic(m) : new _Montgomery(m);
       // TODO(regis): Should we use Barrett reduction for an even modulus?
       var r = new _Bigint();
@@ skipping 284 matching lines @@
   void _sqrTo(_Bigint x, _Bigint r) {
     x._sqrTo(r);
     _reduce(r);
   }
 
   void _mulTo(_Bigint x, _Bigint y, _Bigint r) {
     x._mulTo(y, r);
     _reduce(r);
   }
 }