ES6 transpilation of spread causes CollapseProperties to back off in some cases

For instance, if you have an array of numbers and you call `goog.math.average(...nums)` this translates to (roughly) `goog.math.average.apply(goog.math, nums)` because the ES6 transpilation pass doesn't know that goog.math.average doesn't care what its `this` value is, so it doesn't know that `goog.math.average.apply(null, nums)` would have worked just as well. Then we get a warning "incomplete alias created for namespace goog.math" and CollapseProperties fails to collapse that namespace as well as it otherwise would have.

Maybe we write a pass that goes through after transpilation and replaces the `goog.math` with `null` when we know that can be done safely.