Add support for async/await using Babel

For proof-of-concept, this patch converts a couple of `Promise` returning methods to use `async` instead.
Please note that the `generic` build, based on this patch, has been successfully testing in IE11 (i.e. the viewer loads and nothing is obviously broken).

Being able to use modern JavaScript features like `async`/`await` is a huge plus, but there's one (obvious) side-effect: The size of the built files will increase slightly (unless `SKIP_BABEL == true`). That's unavoidable, but seems like a small price to pay in the grand scheme of things.

Finally, note that the `chromium` build target was changed to no longer skip Babel translation, since the Chrome extension still supports version `49` of the browser (where native `async` support isn't available).

src/shared/message_handler.js

@@ -18,13 +18,11 @@ import {
   ReadableStream, UnexpectedResponseException, UnknownErrorException
 } from './util';
 
-function resolveCall(fn, args, thisArg = null) {
+async function resolveCall(fn, args, thisArg = null) {
   if (!fn) {
-    return Promise.resolve(undefined);
+    return;
   }
-  return new Promise((resolve, reject) => {
-    resolve(fn.apply(thisArg, args));
-  });
+  return fn.apply(thisArg, args);
 }
 
 function wrapReason(reason) {