compiler error for variable use before declaration

[michaelficarra]

From @jashkenas's message on #1121:

Great -- let's add a compiler error for variables being used before they're declared ... in cases where a subsequent declaration would be sure to shadow.

[jashkenas]

I'm afraid this doesn't seem too feasible -- both because we'd have to introduce the notion of keeping track of every variable used in the program, along with their visibility to other scopes ... and because it makes some existing valid programs into errors:

# Here, api is a global variable from another file.
api.object = {}

# Deep inside some inner function later on the page:
-> 
  api = {}