Do not output getter defs when in legacy mode

[IvanSanchez]

This prevents rollup from outputting object getters for reassigned exports (which will trigger a syntax error when Object.freeze() is skipped).

Related to #1068, based on comments from futurist/rollup-plugin-es3#3 , will try to add added a test for this.

[IvanSanchez]

FYI, without the fix, the new test fails as so:

  var browserStuff = (Object.freeze || Object)({
-         get browserSpecificThing () { return browserSpecificThing; },
+         browserSpecificThing: browserSpecificThing,
          foo: foo
  });

[Rich-Harris]

Thank you!

[derrickb]

Hi @Rich-Harris I'm running into this problem now, but there doesn't appear to be a legacy option anymore. Can we reopen this issue? I need getters/setters removed from my output. Thank you!