added collision check for '_this'
Better error recovery for when an arrow function is missing a curly brace.

Also better identification of arrow function expressions.
Addressed code review feedback.
emit 'use strict' at the beginning of the function
Merge pull request #157 from Microsoft/use_strict_in_functions

emit 'use strict' at the beginning of the function
fixed typo in comment
Merge pull request #155 from Microsoft/check_this

added collision check for '_this'
Add error about missing module flag to createProgram
Remove error about missing module flag from checker
Update baselines for not emitting when module flag is not specified
Address PR feedback
Condense all error spans to just the name for variable, class, interface, module, enum and enum member
Complain when a non-void/any function lacks a return expresson.

In effect this fixes #62.

Also
    - Changes the error message for get accessors lacking return expressions.
    - Actually checks for return expressions instead of return statements for get-accessors.
    - Removes fancy quotes.
    - Corrects errors in the compiler caught by the new check.
    - Simplified `checkAndAggregateReturnTypes` by extracting it out to `visitReturnStatements`.
Removed TODO relating to the task I just fixed.
Minor renaming.
Changed check on annotated functions/getters to only look for return statements.