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.

Das Issue nennt keine Repository-Dateien oder Tests. Beginne damit, den Source-Map-Emissionspfad des Compiler-Hosts nachzuverfolgen und zu untersuchen, wie transformierte Quellen gelesen werden. Ermittle anschließend, wo Inline-Maps oder benachbarte .ts.map-Dateien gefunden und zusammengesetzt würden. Als erledigt gilt die Aufgabe, wenn der Compiler beide Eingabe-Map-Formen akzeptiert und sie mit seinen emittierten Source Maps kombiniert.