Support single-module transpilation mode

[mhegazy]

The compiler does not do emit based on semantic information in most cases. this allows us to provide an api for transpiling a module that does not require typecheck or full program information. just single file syntactic transformation.
This can be helpful in multiple scenarios, including: typescript-simple (https://github.com/teppeis/typescript-simple), JSPM (#2233), es6-module-loader and as atom extensions.

A new compiler flag (--singeFileTranspilation or --disableFullProgramOptimizations for instance) that will make cases that break single file-transpilation an error namely: 1.Const enums defined in ambient modules and 2.Re-exporting types-only constructs when targeting ES6

[sophiajt]

👍 though we need a good name for the flag :)

[vladima]

Proposed implementation #2550.
Regarding the name of the flag: we've already had a long discussion about possible names but all options were non-ideal, Some names that we've tried are: 'separateCompilation', 'singleFileEmit', 'disablGlobalOptimizations', 'safeForTranspile', 'isolated', 'singleFileScope' - we could not pick one since all of them were equally bad. What about moving from the boolean flag and instead have an enum named 'compilationMode' or 'compilationUnit' or something similar? Enum values will be 'program' and 'file' and 'program' will be default.

tsc a.ts b.ts --compilationUnit: file