[bug]: Fix broken summarizeEvents function

[sirugh]

Description

A bug in the summarizeEvents function was causing the dev server to exit when switching branches and/or running yarn install. This occurred because the reducer function had malformed arguments and also because the reducer was incorrectly not returning the accumulator value after modifying it.

Related Issue

Closes PWA-405.

Acceptance

Verification Stakeholders

Specification

Verification Steps

Prereq: in watch-all.js change line 81 to eventBuffer.length >= 1 ? summarizeEvents() : eventBuffer; This will cause the function to run so we can test it.

1. Run yarn watch:all on this branch.
2. In a different terminal, switch branches ie git checkout origin/develop~1
3. Run yarn install
4. At no point should the watcher have exited.

Screenshots / Screen Captures (if appropriate)

Yay!

Checklist

• I have updated the documentation accordingly, if necessary.
• I have added tests to cover my changes, if necessary.

[PWAStudioBot]

	Messages
📖	Access a deployed version of this PR here. Make sure to wait for the "pwa-pull-request-deploy" job to complete.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
📖	Associated JIRA tickets: PWA-405.

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