grow method
Managed _array__grow; this reserves space but does not change size.
Implementation
void grow(int count) {
_checkUint32(count, 'count');
final newSize = _checkedSize(_size + count);
if (newSize <= capacity) return;
var newCapacity = capacity * 2;
if (newCapacity < 8) newCapacity = 8;
if (newCapacity < newSize) newCapacity = newSize;
reserve(newCapacity);
}