Fixes workbox dependencies

[arcanis]

cc @jeffposnick @philipwalton (didn't open an issue since both the problem and fix are relatively straightforwards)

Description of what's changed/fixed.

The workbox-webpack-plugin is trying to access the babel-runtime package (because it gets transformed using babel-plugin-transform-runtime), which isn't declared in its dependencies. As a result package managers offer no guarantee as to which version will be provided, or even if babel-runtime will be installed at all.

This diff simply adds the dependency that's missing, based on what I found in the package-lock.json at the root of the repository.

[jeffposnick]

Thanks—this is something that I caught when doing a dependency update sweep in preparation for our v4 release: https://github.com/GoogleChrome/workbox/pull/1658/files#diff-d9674ca55441128e3145e9552ce61a71R31

I believe that we were getting by because workbox-build was pulling in the babel-runtime package implicitly, but it's obviously preferable to have it explicitly listed, as it should be in that PR.

My personal preference would be to just defer to that upcoming change in v4 unless this is something that is actively causing a failure for you with the current v3 release.

[arcanis]

I noticed it while working on facebook/create-react-app#5136, which adds support for Plug'n'Play to create-react-app. Since package boundaries are enforced under this environment, it throws an error:

[jeffposnick]

Gotcha—Okay, we can merge this in for a 3.6.2 release given that.

[jeffposnick]

Thanks—I was just going to ask about the depcheck 😄

[coveralls]

Coverage remained the same at 86.079% when pulling debb46f on arcanis:fixes-workbox-dependencies into 3612461 on GoogleChrome:master.

[arcanis]

Perfect, thanks!

[jeffposnick]

3.6.2 is out with the fix: https://github.com/GoogleChrome/workbox/releases/tag/v3.6.2

Thanks again!