typescript-helpers.js shouldn't be included in sourcemap

[ronzeidman]

This is part of the sourcemap when I use rollup:
"sources":["../node_modules/rollup-plugin-typescript/src/typescript-helpers.js","..
since the rollup-plugin-typescript is a devDependency it won't be included when someone imports my library.
This causes issues with bundling sites that uses my library
Example:
https://github.com/ronzeidman/ng2-ui-auth/blob/master/dist/ng2-ui-auth.js.map

[maxdavidson]

This was an issue with rollup-plugin-babel that was resolved by adding functionality to rollup for ignoring a file from the sourcemap. See rollup/rollup-plugin-babel#64.

[TrySound]

@maxdavidson Thanks. Do you want to create PR with fix and test case?

[maxdavidson]

@TrySound Sure, I'll see what I come up with