Properly make inferences from partial source type

[ahejlsberg]

Fixes #42030.

[DanielRosenwasser]

@typescript-bot pack this

[typescript-bot]

Heya @DanielRosenwasser, I've started to run the tarball bundle task on this PR at 718b3d4. You can monitor the build here.

[typescript-bot]

Hey @DanielRosenwasser, I've packed this into an installable tgz. You can install it for testing by referencing it in your package.json like so:

{
    "devDependencies": {
        "typescript": "https://typescript.visualstudio.com/cf7ac146-d525-443c-b23c-0d58337efebc/_apis/build/builds/91721/artifacts?artifactName=tgz&fileId=BD1C7A363E8AF45FEAC933A438A56E34CD6888008072D2D0722D05E669AA795E02&fileName=/typescript-4.2.0-insiders.20201219.tgz"
    }
}

and then running npm install.

---

There is also a playground for this build and an npm module you can use via "typescript": "npm:@typescript-deploys/pr-build@4.2.0-pr-42038-2".;