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