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, 11 insertions, 0 deletions

diff --git a/test/files/run/t7015.check b/test/files/run/t7015.check
new file mode 100644
index 0000000000..7651fe06b0
--- /dev/null
+++ b/test/files/run/t7015.check
@@ -0,0 +1,11 @@
+Method returns Null type: null
+Method takes non Null type: null
+call through method null
+call through bridge null
+fetch field: null
+fetch field on companion: null
+fetch local: null
+fetch array element: null
+method that takes object: null
+method that takes anyref: null
+method that takes any: null