fix: implement cacheKeyForTree that returns null

[brendenpalmer]

fix: implement cacheKeyForTree that returns null

This fixes a bug between ember-cli-typescript, ember-engines dedupe, and ember-cli-babel. ember-cli-babel uses the existence of ember-cli-typescript as a dependency to determine whether to process *.ts files; however, ember-engines dedupe is stateful so it's possible for ember-cli-typescript to not be a dependency when ember-cli-babel is running.

For more info on ember-engines dedupe logic: https://github.com/ember-engines/ember-engines/blob/master/packages/ember-engines/lib/utils/deeply-non-duplicated-addon.js#L35

For more info on how ember-cli-babel determines whether to process *.ts: https://github.com/babel/ember-cli-babel/blob/master/lib/babel-options-util.js#L407

[dfreeman]

This seems reasonable to me! It's a bit of a shame we need to invalidate the caching here since it would otherwise be totally valid, but since we don't have any trees of runtime code the impact should be pretty minimal.

[brendenpalmer]

@dfreeman I agree. One of the alternatives is to update ember-cli-babel to always transpile *.ts files, which is something I'm exploring in conjunction w/ this. If a fix in ember-cli-babel can land sooner than I expect, then I'll revert this PR (if it's merged by then), or close this out.

[jamescdavis]

@brendenpalmer emberjs/ember-cli-babel#400 resolved this, right?

[brendenpalmer]

@jamescdavis Yes, this has been resolved in ember-cli-babel itself, closing this PR out