[ts] Type-check sources/JavaScriptCompilerPlugin.js with TypeScript
[ts] Type-check sources/JavaScriptCompilerPlugin.js with TypeScript

R=993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org

Bug: chromium:1011811
Change-Id: Icc2e92a70170a014f11f6005ed10e313216d2c81
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2404657
Commit-Queue: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>
Reviewed-by: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend