Convert CommonJS exports to ES6-compatible exports

We use `export default` begrudgingly here. Ideally we'd use just `export`, though this entire SDK expects things to be exported as a default. Instead of breaking everything, we'll sacrifice our export pattern for a smaller diff - a later commit can always do the default export -> regular export conversion.

src/extend.js

@@ -16,11 +16,11 @@ limitations under the License.
 
 'use strict';
 
-module.exports = function(dest, src) {
+export default function(dest, src) {
     for (const i in src) {
         if (src.hasOwnProperty(i)) {
             dest[i] = src[i];
         }
     }
     return dest;
-};
+}