fix(7987): Incremental build works with new trees

[chuckjaz]

• Please check if the PR fulfills these requirements
• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix

• What is the current behavior? (You can also link to an open issue here)
  Incremental rebuild is broken after #7941 #7987
• What is the new behavior (if this is a feature change)?

Incremental building now works.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No

[IgorMinar]

lgtm if CI passes.

please change the commit message to:

build(broccoli): fix incremental builds in broccoli-typescript

Fixes #7987

[mary-poppins]

Merging PR #7989 on behalf of @alxhub to branch presubmit-alxhub-pr-7989.

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