Rotating a vector 90 degrees counter-clockwise from x axis should give a vector pointing at the +y direction, so a [-1,2] would be absurd. You should recheck your math.

Also, the tests require rounding to 10 decimal places, not the nearest integer.

I just tested it, since I couldn't believe that this should work. But actually, the tests are simply botched. They shouldn't use Test.expect but Test.assertSimilar, and they should use random values (not only fixed ones). The .every test is completely flawed if both arrays have different lengths.

I guess that @theintern was just trying to show loopholes in the tests.