Incorrect implementation of PbList.hashcode

[joshua-i]

PbList.hashcode intends to implement the Jenkins hash function. However, in lines like this it is adding before applying the bit shift. Instead of (hash + hash << 10) it should be something like (hash + (hash << 10)).

It was also suggested offline that we should change (...) & 0x3fffffff to 0x3fffffff & (...) , though I don't fully understand why.

[rakudrama]

I recommend:

• use for-in instead of forEach
• copy the exact bit-twiddling from the private class in dart:math
• write arithmetic with the constant first, since this has more precise type inference which leads to better code.