tsc --build mode: "<package> is up to date with its dependencies", but it isn't

[yortus]

TypeScript Version: 3.0.0-dev.20180630

Search Terms:
build mode, up to date, incremental

Steps to Reproduce

Set up @RyanCavanaugh's learn-a sample repo as per his instructions. Run tsc -b packages --verbose to see everything gets built the first time. Now change line 1 in pkg1/src/index.ts to import {} from "./foo"; and save it. Run tsc -b packages --verbose again.

Expected behavior:

pkg2 now fails to build.

Actual behavior:

The compiler reports Project 'packages/pkg2/tsconfig.json' is up to date with .d.ts files from its dependencies, and skips building it, even though we changed pkg1 source in a way that breaks pkg2. If using VSCode, you can see the red squiggle in pkg2/src/index.ts.

Related Issues:

Originally reported in #3469 (comment).

[RyanCavanaugh]

Thanks @yortus !