hashCode for double: difference dart/js

[frankpepermans]

If I run the following code in dart2js or in dartdevc
print((10.4).hashCode);
print((10.8).hashCode);

I get 10 in both cases, seems related to #2884

[matanlurey]

This might be due to performance considerations for Dart2JS/DDC, since JavaScript doesn't have differentiation between int and double (all JavaScript numbers are floating point values).

/cc @rakudrama @jmesserly

[jmesserly]

Good find on #2884! This seems to have been fixed for the VM but not other implementations: 1db8615

We could mix some of the non-integral parts of the JS Number into the hashCode.

[rakudrama]

Fixed via ea6d343

There is one 'num' method for both int and double.

The method is now bigger so no longer gets inlined, so is a little slower for integers.
get:hashCode usually didn't get inlined anyway (e.g. in HashMap implementations it is an interface call).
It is quite a bit slower for non-integers, but presumably this is (1) not very common and (2) there is a benefit from avoiding hash table collisions.