Update PosAnimation.js

Compiler errors:

src/dom/PosAnimation.js line 44 col 18   Identifier '_leaflet_pos' is not in camel case.
src/dom/PosAnimation.js line 52 col 21   Unescaped '-'.
src/dom/PosAnimation.js line 52 col 46   Unescaped '-'.
src/dom/PosAnimation.js line 81 col 18   Identifier '_leaflet_pos' is not in camel case.