Simple control flow analysis

[Vladimir Matveev (vladima)]

This PR adds a basic control flow analysis to typescript compiler. Now it can detect unreachable code, implicit returns of undefined in functions, unused labels, fall- through between cases in switch statements. Some of these checks can be used in #393 or #274.

Disclaimer: This PR is not intended to be checked in as-is. What I rather want is to use it as a starting point to discuss:

• difference between warnings and errors. now all semantic errors that are produced by the compler can be treated as warnings - they do not block emit. However visually they are always presented as errors which might be confusing, I.e. I would expect message about unreachable code to be more like recommendation (presented using green squigglies or shadowed code) instead of being aggressively marked as error
• error suppression story - global and local

[Daniel Rosenwasser (DanielRosenwasser)]

Consider controlFlow.ts, but I like this too.

[Daniel Rosenwasser (DanielRosenwasser)]

Leave a comment to explain that name is optional for the case of implicit labels.

[Daniel Rosenwasser (DanielRosenwasser)]

Consider c.statements.length > 0

[Jason Freeman (JsonFreeman)]

👍

Awesome job!

[Daniel Rosenwasser (DanielRosenwasser)]

We should put this back on the radar again.

[Noel Abrahams (NoelAbrahams)]

We should put this back on the radar again.

👍

[André Werlang (awerlang)]

IMO, it overlaps with TSLint.

Regarding this, I'm concerned with the fact that I cannot use more mature JSHint or ESLint. Perhaps running them on the output (haven't tried that though).
As a TS user, I would prefer that it becomes a full superset of ES6, so I could have back other linters in my workflow.