Two small fixes to fetch

1. Don't ever assign the internal `fetch` to `null`. Instead, throw
   immediately if fetch is not found.
2. Don't extract the global fetch into module state. Instead, export a
   function that looks up the global fetch each time. This allows
   libraries that mock `fetch` to do so after Ember Data has already
   been loaded.

packages/adapter/addon/-private/utils/fetch.ts

@@ -1,17 +1,20 @@
 import require, { has } from 'require';
 
-type MaybeFetch = {
-  (input: RequestInfo, init?: RequestInit | undefined): Promise<Response>;
-} | null;
+type FetchFunction = (input: RequestInfo, init?: RequestInit | undefined) => Promise<Response>;
 
-let _fetch: MaybeFetch = null;
+let _fetch: (() => FetchFunction) | null = null;
 
 if (has('fetch')) {
   // use `fetch` module by default, this is commonly provided by ember-fetch
-  _fetch = require('fetch').default;
+  let foundFetch = require('fetch').default;
+  _fetch = () => foundFetch;
 } else if (typeof fetch === 'function') {
   // fallback to using global fetch
-  _fetch = fetch;
+  _fetch = () => fetch;
+} else {
+  throw new Error(
+    'cannot find the `fetch` module or the `fetch` global. Did you mean to install the `ember-fetch` addon?'
+  );
 }
 
 export default _fetch;

packages/adapter/addon/rest.js

@@ -1057,7 +1057,15 @@ const RESTAdapter = Adapter.extend(BuildURLMixin, {
   },
 
   _fetchRequest(options) {
-    return fetch(options.url, options);
+    let fetchFunction = fetch();
+
+    if (fetchFunction) {
+      return fetchFunction(options.url, options);
+    } else {
+      throw new Error(
+        'cannot find the `fetch` module or the `fetch` global. Did you mean to install the `ember-fetch` addon?'
+      );
+    }
   },
 
   _ajax(options) {