As a MetaMask developer, I want to prepopulate state with specific fixture data when running yarn start and yarn dist

As a MetaMask developer, I want to prepopulate state with specific fixture data when running yarn start and yarn dist

This can be very useful for speed of development: I don't need to spend time getting my local stall into the state I need to test my feature, I can just preload with a command

This can improve effectiveness of developers manual testing of their own work: this makes it easy to load different scenarios when developing, and develop against user states that more closely reflect real world usage

This can help with release QA: people testing releases could run `yarn dist` with different fixtures, and quickly get into the state they want to be in to test various edge cases

The most straightforward way to achieve this is a flag on `yarn start` and `yarn dev` that you can pass a json fixture file, and that json will populate the metamask state on background initialization

Something else we could consider, perhaps as a v2, is a feature only available in local development build, that allows uploading a json file from within the wallet UI, which would completely overwrite state, so that rebuilds are not needed to switch between different testing scenarios.

Start by tracing the yarn start, yarn dev, and yarn dist entry points and where background initialization populates MetaMask state. Determine how a JSON fixture would be selected and loaded, and define the completion criteria for preloading state in each command; the optional wallet UI upload is a separate possible phase.

Written by the indexing model from the issue text.