Initialize the map of primitive methods (for GenJSCode)
Init the map of primitive methods for Scala.js (for PrepJSInterop)
Only call from PrepJSInterop.
Only call from PrepJSInterop. In GenJSCode, use scalaPrimitives.isPrimitive instead
For a primitive, is it one for which we should emit its body anyway?
Extension of ScalaPrimitives for primitives only relevant to the JS backend