Move Browserify transforms to gulpfile (#6768)

The flat tests also rely upon these transformations, yet invoke
browserify from the command line rather than using the gulpfile. The
transformations have been specified on the command line for those
instead.

Of course it's not ideal to have the same transformations listed in two
different places, but the plan is to delete the flat tests soon anyway,
so this should suffice until then.

Closes #4538

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