Fix #3383: Add regression test

Author: nicolasstucki

compiler/src/dotty/tools/dotc/CompilationUnit.scala

@@ -1,7 +1,6 @@
 package dotty.tools
 package dotc
 
-import core.Types.Type // Do not remove me #3383
 import util.SourceFile
 import ast.{tpd, untpd}
 import tpd.{ Tree, TreeTraverser }