Allow Closure Compiler to support ES5 code

[dchambers]

Closure Compiler currently errors if you use certain ES5 features, like getter properties. By adding the following line of code at line 74 of ClosureMinifierPlugin:

options.setLanguageIn(LanguageMode.ECMASCRIPT5);

Closure Compiler will start to accept additional ES5 features.

It's my belief that ECMASCRIPT5 mode will not prevent people from using any ES3 features as there is a separate ECMASCRIPT5_STRICT mode that presumably adds these additional restrictions. Obviously we'll need some test(s) to verify this fix.

[vmus]

Is this change still planned? Now that IE8 is no longer supported.