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).

external/builder/preprocessor2.js

@@ -288,6 +288,7 @@ function preprocessPDFJSCode(ctx, code) {
     },
   };
   var parseOptions = {
+    ecmaVersion: 8,
     locations: true,
     sourceFile: ctx.sourceFile,
     sourceType: 'module',