Assignments don't always change variable types

[jakebailey]

Here, a is initially a list of strings, but after line 4, it should be known to be a list of ints.

Found in #334.

[MikhailArkhipov]

This is not currently possible as analysis is static. Code does not track type by location. Thus, at best, variable will have two types. In order to make this work the engine either has to eval code to the completion point (JavaScript editor used to do this) or types attached to variable need to have 'applicable spans' associated with them.

[jakebailey]

Unfortunately I think reassigning variables with different types is a pretty normal thing to do in a non-static language like Python, and it's probably worthwhile to try to do this.

[MikhailArkhipov]

It would be a fundamental and pretty expensive change.

[MikhailArkhipov]

New analysis uses the last assignment. Location tracking folding into #682