IE11 incompatibility in BooleanParameterGroup.js

[tephenavies]

terriajs/lib/Models/BooleanParameterGroup.js

Lines 35 to 36 in 30470f4

	this.OneForAll = new Proxy({}, {});
	this.ParameterList = new Proxy([], {});

This fails in IE11 because Proxy is an ES6 feature that cannot be polyfilled.

Tested with http://ci.terria.io/master/#clean&https://gist.githubusercontent.com/Jason-DNetto/7ccde078dd517be7b279855fdf241cbc/raw/d90452f1789dc68e85e7c1d933f266931262f0b4/checkbox_groups.json

Error:

Error from updateApplicationUrl: ReferenceError: 'Proxy' is undefined

[Jason-DNetto]

Was under the impression that microsoft edge was supposed to replace IE.
What should be done about that error?