SI-7015 Removes redundant aconst_null; pop; aconst_null creation

In an effort to adapt methods and field accesses of type Null to
other types, we were always emitting

    aconst_null
    pop
    aconst_null

The problem is we were doing that even when the JVM was in a position
to know it had null value, e.g. when the user had written a null
constant. This commit fixes that and includes a test to show that the
resulting byte code still works even without repeating ourselves and/or
repeating ourselves.

This commit also makes the scala.runtim.Null$ constructor private. It
was a sealed abstract class which prevented subclassing in Scala, but
it didn't prevent subclassing in Java. A private constructor takes care
of that hole so now the only value of type Null$ should be null.

Along the way I found some other questionable things in adapt and I've
added TODO's and issue https://issues.scala-lang.org/browse/SI-7159 to
track.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/library/scala/runtime/Null$.scala b/src/library/scala/runtime/Null$.scala
index 797b31583d..25b797a606 100644
--- a/src/library/scala/runtime/Null$.scala
+++ b/src/library/scala/runtime/Null$.scala
@@ -11,6 +11,7 @@ package scala.runtime
 /**
  * Dummy class which exist only to satisfy the JVM. It corresponds to
  * `scala.Null`. If such type appears in method signatures, it is erased
- * to this one.
+ * to this one. A private constructor ensures that Java code can't create
+ * subclasses. The only value of type Null$ should be null
  */
-sealed abstract class Null$
+sealed abstract class Null$ private ()