wip
Fix #1172: Also run checksizes over gzipped sizes
Merge pull request #1186 from gzm0/gzip-checksizes

Fix #1172: Also run checksizes over gzipped sizes
Reorganize the construction of ScalaJS in scalajsenv.js.

Instead of building one big literal object, we add properties
one at a time. This doesn't make a difference to Closure, but
now we can define later fields in terms of earlier fields.
In toolsJS test, do not inherit __ScalaJSEnv.

Otherwise, multiple ScalaJS objects will share their env
field, each will try to set things in it and freeze it,
which is no good.