Provide a way to easily mock fetch and Web platform polyfills

package.json

@@ -50,11 +50,11 @@
     "globalize": "1.4.0",
     "immutable": "3.8.2",
     "intersection-observer": "0.4.2",
+    "isomorphic-fetch": "^2.2.1",
     "pepjs": "0.4.2",
     "resize-observer-polyfill": "1.5.0",
     "tslib": "1.8.1",
-    "web-animations-js": "2.3.1",
-    "whatwg-fetch": "3.0.0"
+    "web-animations-js": "2.3.1"
   },
   "devDependencies": {
     "@dojo/loader": "^2.0.0",

src/shim/fetch.ts

@@ -1,5 +1,24 @@
-import global from './global';
+import dojoGlobal from './global';
 `!has('build-elide')`;
-import 'whatwg-fetch';
+import 'isomorphic-fetch';
 
-export default global.fetch.bind(global) as (input: RequestInfo, init?: RequestInit) => Promise<Response>;
+import has from '../has/has';
+
+if (typeof global !== 'undefined' && (global as any).fetch && (global as any).fetch !== dojoGlobal.fetch) {
+	dojoGlobal.fetch = (global as any).fetch;
+}
+
+const _fetch = dojoGlobal.fetch.bind(dojoGlobal) as (input: RequestInfo, init?: RequestInit) => Promise<Response>;
+let replacement: (input: RequestInfo, init?: RequestInit) => Promise<Response>;
+
+export default function fetch(input: RequestInfo, init?: RequestInit) {
+	return replacement ? replacement(input, init) : _fetch(input, init);
+}
+
+export function replace(fetch: (input: RequestInfo, init?: RequestInit) => Promise<Response>) {
+	if (has('test')) {
+		replacement = fetch;
+	} else {
+		throw new Error('Replacement functionality is only available in a test environment');
+	}
+}