Accept Input Source Maps

The typescript compiler produces sources maps mapping from the javascript it produces to the typescript sources it compiled. This is great, but doesn't go far enough if there are tools which generate the sources that are consumed by the typescript compiler.

For example, Tsickle preprocesses typescript sources before passing them to the typescript compiler. As things stand we've had to implement a hack to get our source maps to work. We use a compiler host which performs the tsickle transforms as the typescript compiler reads files, saves the resulting source maps and then compose them with the source maps produced by the typescript compiler as they're emitted.

Instead the typescript compiler could accept source maps as input alongside sources, and compose the input source maps with the source maps produced by the compiler.

The ideal api would be that the typescript compiler would look for either a. an inline source map as a comment in the source file or b. a file in the same directory with the name {source file name}.ts.map, which shouldn't require any crazy file resolution.

El issue no menciona archivos ni tests del repositorio. Empieza rastreando la ruta de emisión de source maps del compiler host y cómo se leen las fuentes transformadas; después, determina dónde se descubrirían y compondrían los mapas inline o los archivos .ts.map adyacentes. Se considera terminado cuando el compilador acepta ambas formas de mapas de entrada y las combina con sus source maps emitidos.