Avoid NotDefinedHere error in FirstTransform.

We saw a NotDefinedHere error for `dotc t2310.scala`:

demanding denotation of module class None$ at phase firstTransform(3) outside
defined interval: defined periods are Period(4..8, run = 2) Period(1..2, run = 2)

This is weird. Period 3 (which is FirstTransform) was missing. I don't know why, but
the problem goes away if we make InstChecks an IdentityTransformer. @DarkDimius:
Do you have an idea why this could be? Is there soemthing which requires
FirstTransform immediately after Typer?

Author: odersky

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

@@ -9,9 +9,10 @@ import Types._, Symbols._, Flags._, StdNames._
 import util.Positions._
 import ast.Trees._
 import typer.ErrorReporting._
+import DenotTransformers._
 
 /** This checks `New` nodes, verifying that they can be instantiated. */
-class InstChecks extends Phase {
+class InstChecks extends Phase with IdentityDenotTransformer {
   import ast.tpd._
 
   override def phaseName: String = "instchecks"