Remote Inspector should warn if the browser can't support JS ES6 features

[elecro]

Currently the Remote Inspector uses a few JS ES6 features which are not (yet) supported by other browsers. We should give the users a warning about that the Inspector won't work if they don't have the given JS feature.

Some of the features which we should test:

• Map object.
• for...of statement
• ??

For testing the JS features maybe we could use eval('..') code snippets

Other things to consider:

• -webkit prefixed CSS attributes.