Revert modPow introduction since dart2js does not like it.

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
===================================================================
--- runtime/lib/bigint.dart	(revision 42825)
+++ runtime/lib/bigint.dart	(working copy)
@@ -1328,6 +1328,8 @@
     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);