Added implementation of TypeTestsCasts#box

Author: odersky

src/dotty/tools/dotc/transform/TypeTestsCasts.scala

@@ -13,6 +13,7 @@ import core.StdNames._
 import core.transform.Erasure.isUnboundedGeneric
 import typer.ErrorReporting._
 import ast.Trees._
+import Erasure.Boxing.box
 
 /** This transform normalizes type tests and type casts.
  *  Any remaining type tests
@@ -23,8 +24,6 @@ import ast.Trees._
 class TypeTestsCasts extends TreeTransform {
   import ast.tpd._
 
-  def box(tree: Tree): Tree = ???
-
   override def name: String = "typeTestsCasts"
 
   override def transformTypeApply(tree: TypeApply)(implicit ctx: Context, info: TransformerInfo): Tree = ctx.traceIndented(s"transforming ${tree.show}", show = true) {