Fix #8936: Correct generic signature for Nothing/Null

The runtime classes are called "scala.runtime.Nothing$" and
"scala.runtime.Null$", but the signatures were missing the dollar at the
end. Since these two types are special anyway, we can just emit their
signature directly and not bother with loading their classfiles.

Author: smarter

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -355,13 +355,11 @@ class Definitions {
   @tu lazy val NothingClass: ClassSymbol = enterCompleteClassSymbol(
     ScalaPackageClass, tpnme.Nothing, AbstractFinal, List(AnyClass.typeRef))
   def NothingType: TypeRef = NothingClass.typeRef
-  @tu lazy val RuntimeNothingModuleRef: TermRef = ctx.requiredModuleRef("scala.runtime.Nothing")
   @tu lazy val NullClass: ClassSymbol = {
     val parent = if (ctx.explicitNulls) AnyType else ObjectType
     enterCompleteClassSymbol(ScalaPackageClass, tpnme.Null, AbstractFinal, parent :: Nil)
   }
   def NullType: TypeRef = NullClass.typeRef
-  @tu lazy val RuntimeNullModuleRef: TermRef = ctx.requiredModuleRef("scala.runtime.Null")
 
   /** An alias for null values that originate in Java code.
    *  This type gets special treatment in the Typer. Specifically, `UncheckedNull` can be selected through:

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -202,9 +202,9 @@ object GenericSignatures {
           else if (sym == defn.UnitClass || sym == defn.BoxedUnitModule)
             jsig(defn.BoxedUnitClass.typeRef)
           else if (sym == defn.NothingClass)
-            jsig(defn.RuntimeNothingModuleRef)
+            builder.append("Lscala/runtime/Nothing$;")
           else if (sym == defn.NullClass)
-            jsig(defn.RuntimeNullModuleRef)
+            builder.append("Lscala/runtime/Null$;")
           else if (sym.isPrimitiveValueClass)
             if (!primitiveOK) jsig(defn.ObjectType)
             else if (sym == defn.UnitClass) jsig(defn.BoxedUnitClass.typeRef)

tests/pos-java-interop/i8936/A.scala

@@ -0,0 +1,3 @@
+class A {
+  def no: List[Nothing] = Nil
+}

tests/pos-java-interop/i8936/B.java

@@ -0,0 +1,8 @@
+import scala.collection.immutable.List;
+
+class B {
+  void Nothingness(){
+    A a = new A();
+    List<scala.runtime.Nothing$> strings = a.no();
+  } 
+}