Improve _toInt64() and _toUint64() conversions in TypedData.

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
===================================================================
--- runtime/lib/bigint.dart	(revision 43696)
+++ runtime/lib/bigint.dart	(working copy)
@@ -1167,7 +1167,7 @@
   // This method must support smi._toBigint()._shrFromInt(int).
   int _shrFromInt(int other) {
     if (_used == 0) return other;  // Shift amount is zero.
-    if (_neg) throw "negative shift amount";  // TODO(regis): What exception?
+    if (_neg) throw new RangeError(this);
     assert(_DIGIT_BITS == 32);  // Otherwise this code needs to be revised.
     var shift;
     if ((_used > 2) || ((_used == 2) && (_digits[1] > 0x10000000))) {
@@ -1186,7 +1186,7 @@
   // An out of memory exception is thrown if the result cannot be allocated.
   int _shlFromInt(int other) {
     if (_used == 0) return other;  // Shift amount is zero.
-    if (_neg) throw "negative shift amount";  // TODO(regis): What exception?
+    if (_neg) throw new RangeError(this);
     assert(_DIGIT_BITS == 32);  // Otherwise this code needs to be revised.
     var shift;
     if (_used > 2 || (_used == 2 && _digits[1] > 0x10000000)) {
@@ -1345,8 +1345,10 @@
     if (e < 0) throw new RangeError(e);
     if (m <= 0) throw new RangeError(m);
     if (e == 0) return 1;
+    e = e._toBigint();
+    m = m._toBigint();
     final m_used = m._used;
-    final m_used2p2 = 2*m_used + 1 + 1;  // +1 for leading zero.
+    final m_used2p2 = 2*m_used + 2;
     final e_bitlen = e.bitLength;
     if (e_bitlen <= 0) return 1;
     if ((e is! _Bigint) || m.isEven) {
@@ -1353,7 +1355,6 @@
       _Reduction z = (e_bitlen < 8 || m.isEven) ?
           new _Classic(m) : new _Montgomery(m);
       // TODO(regis): Should we use Barrett reduction for an even modulus?
-      var m_used = m._used;
       var r_digits = new Uint32List(m_used2p2);
       var r2_digits = new Uint32List(m_used2p2);
       var g_digits = new Uint32List(m_used + (m_used & 1));
@@ -1630,7 +1631,8 @@
     while (x_used > 0 && x_digits[x_used - 1] == 0) {
       --x_used;
     }
-    x_used = _Bigint._drShiftDigits(x_digits, x_used, m_used, x_digits);
+    // Shift right by m_used digits or, if processing pairs, by i (even) digits.
+    x_used = _Bigint._drShiftDigits(x_digits, x_used, i, x_digits);
     if (_Bigint._compareDigits(x_digits, x_used, m_digits, m_used) >= 0) {
       _Bigint._absSub(x_digits, x_used, m_digits, m_used, x_digits);
     }
@@ -1700,7 +1702,7 @@
     var digits;
     var used;
     if (x._neg || x._compare(_m) >= 0) {
-      var r = x.rem(_m);
+      var r = x._rem(_m);
       if (x._neg && !r._neg && r._used > 0) {
         r = _m._sub(r);
       }
@@ -1711,7 +1713,7 @@
       used = x._used;
       digits = x._digits;
     }
-    var i = used + (used + 1);  // Copy leading zero if any.
+    var i = used + (used & 1);  // Copy leading zero if any.
     while (--i >= 0) {
       r_digits[i] = digits[i];
     }