Add modPow(int exponent, int modulus) method to int abstract class.

sdk/lib/core/int.dart

Index: sdk/lib/core/int.dart
===================================================================
--- sdk/lib/core/int.dart	(revision 43571)
+++ sdk/lib/core/int.dart	(working copy)
@@ -103,6 +103,14 @@
    */
   int operator >>(int shiftAmount);
 
+  /**
+   * Returns this integer to the power of [exponent] modulo [modulus].
+   *
+   * The [exponent] must be non-negative and [modulus] must be
+   * positive.
+   */
+  int modPow(int exponent, int modulus);
+
   /** Returns true if and only if this integer is even. */
   bool get isEven;