lib.d.ts defines "postMessage" for window, but web worker scripts have a different signature

[xirzec]

Webworkers have a global "postMessage(message:any):void" method, but TS uses the "postMessage(message:any, origin:string, ports?:any):void" definition from window.postMessage.

This causes something like

postMessage("Hello");

to be detected as an error, even though it is valid from inside a webworker. Maybe there needs to be a way to indicate to TS (via a comment in the file?) that the current script is intended to be loaded inside a webworker.

[mhegazy]

We have a different library file for webworkers, under lib.webworkers.d.ts. this should have the correct definition. currently there is no way to reference this file except using the full path, which is not ideal. adding a commandline switch to specify which flavor of the library you want is tracked under #494