Test if ES6 is ~fully supported

supportsES6.js

var supportsES6 = function() {
  try {
    new Function("(a = 0) => a");
    return true;
  }
  catch (err) {
    return false;
  }
}();

MediaWiki previously used this snippet and is now updating it to also test for these ES2017 features:

• Trailing commas in function parameters
• Async functions

See the update in patchset 1141401.

Ensuring ES2015 (ES6) grammar and syntax support:

new Function( '(a = 0) => a' );

Ensuring ES2017 (ES8) grammar and syntax support:

new Function( 'async (a = 0,) => a' );

The crucial test is the one for trailing commas, as it was the last ES2017 feature to be implemented (for instance, in Chrome 58).
You can find more details in the "Feature summary" table at the end of this Feature support list on caniuse.com.