Add two TODOs

Author: odersky

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -818,6 +818,7 @@ trait Checking {
   def checkImplicitConversionUseOK(tree: Tree)(using Context): Unit =
     val tree1 = if Nullables.unsafeNullsEnabled then
       // If unsafeNulls is enabled, a cast and a closure could be added to the original tree
+      // !!! TODO: We need a test for that. Right now everything passes even if this case is commented out.
       stripCast(closureBody(tree))
     else tree
     val sym = tree1.symbol

compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala

@@ -161,6 +161,7 @@ object ErrorReporting {
           case fail: Implicits.NoMatchingImplicits => // do nothing
           case _ => attempts += ((failure.tree, ""))
       if foundWithoutNull then
+        // !!! TODO: The need a test that tests this error message with a check file
         i""".
           |Since explicit-nulls is enabled, the selection is rejected because
           |${qualType.widen} could be null at runtime.