Ember 3.27+ issues deprecation for accessing `window.Ember`

[rwjblue]

The code here:

ember-fetch/assets/browser-fetch.js.t

Line 4 in eaf18b6

var Promise = originalGlobal.Ember.RSVP.Promise;

Accesses window.Ember, which on Ember 3.27 and higher issues a deprecation.

[rwjblue]

I think the best fix for this (that would support both older Ember and newer) would be to change our .transpileTree invocation to actually transpile modules for us:

ember-fetch/index.js

Lines 124 to 128 in eaf18b6

	browserTree = debug(babelAddon.transpileTree(browserTree, {
	'ember-cli-babel': {
	compileModules: false,
	},
	}), 'after-babel');

Then we'd change our assets/browser-fetch.js.t to just use ES6 imports for Ember.