Enable loose mode for ES6 class transformation

Loose mode is required to make inheritance work in IE 9-10:
http://babeljs.io/docs/usage/caveats/

Change-Id: I23e954fd02fbaa2327e33ad0f8091306da2f6667
Closes-Bug: #1602738

webpack.config.js

@@ -20,7 +20,11 @@ module.exports = {
         exclude: [/node_modules\//, /\/expression\/parser\.js$/],
         query: {
           cacheDirectory: true,
-          plugins: ['transform-runtime', 'transform-es2015-modules-commonjs'],
+          plugins: [
+            ['transform-es2015-classes', {loose: true}],
+            'transform-runtime',
+            'transform-es2015-modules-commonjs'
+          ],
           presets: ['es2015-webpack', 'react']
         }
       },