Fix memory leak

The lazy val `dummyTree` acquires a type because of
copy-on-write and that type can refer via lastDenotation
to a context base.

Author: odersky

src/dotty/tools/dotc/typer/ProtoTypes.scala

@@ -477,7 +477,7 @@ object ProtoTypes {
     def apply(tp: Type) = wildApprox(tp, this)
   }
 
-  @sharable private lazy val dummyTree = untpd.Literal(Constant(null))
+  @sharable private def dummyTree = untpd.Literal(Constant(null))
 
   /** Dummy tree to be used as an argument of a FunProto or ViewProto type */
   object dummyTreeOfType {