I understand that this is an old question, but maybe this will help someone. I assume you are using Scala.
Here is what I found after some diving code. Adding the following lines to your project settings in project/Build.scala should include some additional features:
closureCompilerOptions += "advancedOptimizations", closureCompilerOptions += "checkControlStructures", closureCompilerOptions += "checkTypes", closureCompilerOptions += "checkSymbols"
There is also checkCaja ; they are defined in play.core.jscompile.JavascriptCompiler
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