remove all uses of element.node

[sigmundch]

element.node is not cached, it can trigger reparsing ASTs and remapping nodes to their elements. It's recommended that we don't use it.

The CL at https://chromereviews.googleplex.com/143177014/ gets rid of most of the uses except 2:

• driving the codegen: this can be fixed easily once we change the compiler to have a pipeline (check-generate, check-generate). The logic in the checker for visiting units would then be enough to handle this
• determine some information about constant fields from the js codegen. I still need to look further into this one.

[jmesserly]

second one is fixed in https://chromereviews.googleplex.com/147927013/
edit: landed as b1b36fe

[sigmundch]

and the former one will be fixed here: https://chromereviews.googleplex.com/145167013/

[jmesserly]

since both fixes landed, assuming this is now closed :)