Properly shade com.google.code.gson:gson

The "shaded" JAR of closure-compiler includes all the classes in `com.google.gson` without shading (renaming). This can lead to confusing errors far downstream - for example, I came upon this issue when [sbt-dependency-check](https://github.com/albuch/sbt-dependency-check) raised a `NoMethodError`, which turned out to be caused by my project using [scala-js](https://github.com/scala-js/scala-js). I am [definitely](https://github.com/google/closure-compiler/issues/2815) [not](https://github.com/google/closure-compiler/issues/2562) [the first](https://github.com/google/closure-compiler/issues/2120) one to experience this.

Is there a reason why `com.google.gson` is bundled rather than shaded? What is the "preferred" way to deal with such problems downstream?