self is not defined

[shanejonas]

Describe the bug
I have a nested dependency that relies on isomorphic-fetch, which i'm unable to load inside SES due to self not being defined.

To Reproduce
Steps to reproduce the behavior:

1. Use snaps-cli to init a new project
2. Add const supported = "URLSearchParams" in self; to the code
3. See error

Expected behavior
As defined in the spec, self should exist and be the window object.

The window, frames, and self attributes' getters must return this Window object's relevant Realm.[[GlobalEnv]]'s EnvironmentRecord's [[GlobalThisValue]].

Source: https://html.spec.whatwg.org/multipage/window-object.html#dom-self

Screenshots

Browser details (please complete the following information):

• OS: OS X
• Browser: chrome
• MetaMask Version: develop

Additional context
JakeChampion/fetch#657 (comment)