Prevent __proto__ pollution in util.deepExtend

[Feiyang1]

No description provided.

[changeset-bot]

🦋 Changeset detected

Latest commit: 0df9205

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 28 packages

Name	Type
@firebase/util	Patch
@firebase/analytics	Patch
@firebase/app	Patch
@firebase/component	Patch
@firebase/database	Patch
firebase	Patch
@firebase/firestore	Patch
@firebase/installations	Patch
@firebase/messaging	Patch
@firebase/performance	Patch
@firebase/remote-config	Patch
@firebase/rules-unit-testing	Patch
@firebase/storage	Patch
@firebase/functions	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/util

  Type	Base (a9ba7ba)	Head (a7e6c0a)	Diff
  browser	20.1 kB	20.2 kB	+69 B (+0.3%)
  esm2017	18.9 kB	19.0 kB	+69 B (+0.4%)
  main	21.3 kB	21.3 kB	+69 B (+0.3%)
  module	20.1 kB	20.2 kB	+69 B (+0.3%)

• firebase

  Type	Base (a9ba7ba)	Head (a7e6c0a)	Diff
  firebase-app.js	20.0 kB	20.0 kB	+17 B (+0.1%)
  firebase-database.js	190 kB	190 kB	+17 B (+0.0%)
  firebase-performance-standalone.es2017.js	71.7 kB	71.7 kB	+18 B (+0.0%)
  firebase-performance-standalone.js	48.1 kB	48.1 kB	+17 B (+0.0%)
  firebase.js	820 kB	820 kB	+17 B (+0.0%)

Test Logs

• Base (a9ba7ba7): https://github.com/firebase/firebase-js-sdk/actions/runs/329961119
• Head (a7e6c0a9): https://github.com/firebase/firebase-js-sdk/actions/runs/332264048

[schmidt-sebastian]

2 nits but LGTM.

[schmidt-sebastian]

Maybe a little cleaner if you create a type alias for Record<string, unknown> to reduce the repetitive cast.