Remove unused method traverseEs6

[thheller]

fee5195

Removes the NodeTraversal.traverseEs6 static function which I guess got refactored to be NodeTraversal.traverse now (which it also delegates to). I'm using it some shadow-cljs utility passes.

Please consider keeping the NodeTraversal.traverseEs6 function around for a few versions since it creates unnecessary breakage otherwise. I will update my code but can't because NodeTraversal.traverse is not available yet.

Just removing the function without deprecating it first (it isn't deprecated in 20180402) makes upgrading a bit painful.

[MatrixFrog]

Ah sorry. We usually do keep it around for one version but I forgot to do it in this case. Would using one the "snapshot" builds from https://oss.sonatype.org/content/repositories/snapshots/com/google/javascript/closure-compiler/1.0-SNAPSHOT/ help?

[thheller]

The problem I have is that removing this function breaks backward compatibility. Sometimes my users prefer to stick to specific versions because upgrading breaks their build (eg. #2822).

[MatrixFrog]

Yeah, we'll put it back, at least for one release. My bad.