Update FetchHttpClient to make fetch function optional. (#1253)

lib/net/FetchHttpClient.js

@@ -3,9 +3,12 @@
 const {HttpClient, HttpClientResponse} = require('./HttpClient');
 
 /**
- * HTTP client which uses a `fetch` function to issue requests. This fetch
- * function is expected to be the Web Fetch API function or an equivalent, such
- * as the function provided by the node-fetch package (https://github.com/node-fetch/node-fetch).
+ * HTTP client which uses a `fetch` function to issue requests.
+ *
+ * By default relies on the global `fetch` function, but an optional function
+ * can be passed in. If passing in a function, it is expected to match the Web
+ * Fetch API. As an example, this could be the function provided by the
+ * node-fetch package (https://github.com/node-fetch/node-fetch).
  */
 class FetchHttpClient extends HttpClient {
   constructor(fetchFn) {
@@ -36,7 +39,8 @@ class FetchHttpClient extends HttpClient {
     );
     url.port = port;
 
-    const fetchPromise = this._fetchFn(url.toString(), {
+    const fetchFn = this._fetchFn || fetch;
+    const fetchPromise = fetchFn(url.toString(), {
       method,
       headers,
       body: requestData || undefined,