x/playground: error line highlighting broken with multi-file programs

[bradfitz]

With the new support for multiple files (#32040), the error highlighting is broken (sometimes):

https://play.golang.org/p/S5z9R-mbAzj

/cc @ysmolsky @dmitshur

[dmitshur]

Previously, there was only one file and so the line number was the only thing that mattered. Now, we'll need to have logic that maps a file:line pair to a single line number of the combined txtar file.

That logic can be either in the frontend or backend (but backend is probably easier, since it's Go code).

[gopherbot]

Change https://golang.org/cl/180117 mentions this issue: playground: support error line highlighting for multi-file program