Strenghten postCondition of firstTransform

All tests pass, but good to have the condition in there.

Author: odersky

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

@@ -35,6 +35,9 @@ class FirstTransform extends MiniPhaseTransform with IdentityDenotTransformer {
   override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = tree match {
     case Select(qual, _) if tree.symbol.exists =>
       assert(qual.tpe derivesFrom tree.symbol.owner, i"non member selection of ${tree.symbol.showLocated} from ${qual.tpe}")
+    case _: TypeTree =>
+    case _: Import | _: NamedArg | _: TypTree =>
+      assert(false, i"illegal tree: $tree")
     case _ =>
   }