setUpper2x2 method
Sets the upper 2x2 of the matrix to be arg.
Implementation
void setUpper2x2(Matrix2 arg) {
  final argStorage = arg._m2storage;
  _m4storage[0] = argStorage[0];
  _m4storage[1] = argStorage[1];
  _m4storage[4] = argStorage[2];
  _m4storage[5] = argStorage[3];
}