VM: Remove _leftShiftWithMask32.

runtime/lib/bigint.dart

Index: runtime/lib/bigint.dart
diff --git a/runtime/lib/bigint.dart b/runtime/lib/bigint.dart
index ae941b756fdc3682221dc1f54944e5eabe03e0de..bc9b8f486dea198aff173f0822e0584326bfd940 100644
--- a/runtime/lib/bigint.dart
+++ b/runtime/lib/bigint.dart
@@ -1336,16 +1336,6 @@ class _Bigint extends _IntegerImplementation implements int {
     return str;
   }
 
-  _leftShiftWithMask32(int count, int mask) {
-    if (_used == 0) return 0;
-    if (count is! _Smi) {
-      _shlFromInt(count);  // Throws out of memory exception.
-    }
-    assert(_DIGIT_BITS == 32);  // Otherwise this code needs to be revised.
-    if (count > 31) return 0;
-    return (_digits[0] << count) & mask;
-  }
-
   int _bitAndFromInteger(int other) {
     return other._toBigint()._and(this)._toValidInt();
   }