Implement modInverse (bigint version does not support even modulus yet).

sdk/lib/core/int.dart

Index: sdk/lib/core/int.dart
diff --git a/sdk/lib/core/int.dart b/sdk/lib/core/int.dart
index 1d29a728f0399e2c16059fa1bb0ec26b69d29c8b..f12c97c5edb158fdcc63f2a46f4e93780c5c7ead 100644
--- a/sdk/lib/core/int.dart
+++ b/sdk/lib/core/int.dart
@@ -111,6 +111,14 @@ abstract class int extends num {
    */
   int modPow(int exponent, int modulus);
 
+  /**
+   * Returns the modular multiplicative inverse of this integer
+   * modulo [modulus].
+   *
+   * The [modulus] must be positive.
+   */
+  int modInverse(int modulus);
+
   /** Returns true if and only if this integer is even. */
   bool get isEven;