Library Affected:
workbox-build

Browser & Platform:
n/a

Issue or Feature Request Description:
It looks like source map support was added for the Webpack plugin in #2239 when injecting the manifest. However, there is no such support for the injectManifest method in the workbox-build package. It would be helpful if the same code could be applied there, i.e. if a source map exists for swSrc then write an adjusted one for swDest.