Add -D js-es5.

We can generate smaller and faster JS if we don't care about IE8.

Here's the compatibility table:
http://kangax.github.io/es5-compat-table
add support for dates in json serializer
no cast
[java/cs] Fix special Lib functions
[java] Fix -java-lib enum. Closes #2410
[cs] Only use native properties for interacting with CsNative
[cs] Starting to add unit tests to net-lib
add ExprTools.getValue
[cs] inner classes support
[cs] generics support
[cs] enum generation
[cs] WIP: overriding native properties
[cpp] Add hxcpp_api_level to allow backwards compatibility in both haxe and c++ code.  Add additional memory usage statistics
[cpp] Minimise the number of calls to super when marking/visiting.  Mark appropriate objects as having no external references.
make the `replacer` argument for JsonPrinter.print optional.
Merge pull request #2669 from HaxeFoundation/js-es5

Add -D js-es5.
respect windows \ paths in Path.normalize
change Path.join to accept an Array<String>
move date compare lower on checks
Merge pull request #2680 from Blank101/jsondate

Add support for dates in json serializer