Mixin: implementations of Scala2 LazyVals need a rhs.

Unlike normal fields, they aren't initialised in $init method, and need to know a bit about class layout of implementation class.
Luckily AugmentScala2Traits added just enough symbols to make this work.

Author: DarkDimius

src/dotty/tools/dotc/transform/Mixin.scala

@@ -163,7 +163,13 @@ class Mixin extends MiniPhaseTransform with SymTransformer { thisTransform =>
         if (isCurrent(getter) || getter.is(ExpandedName))
           // transformFollowing call is needed to make memoize & lazy vals run
           transformFollowing(
-            DefDef(implementation(getter.asTerm), if (isScala2x) default else initial))
+            DefDef(implementation(getter.asTerm), 
+               if (isScala2x) {
+                 if (getter.is(Flags.Lazy)) // lazy vals need to have a rhs that will be the lazy initializer
+                   ref(mixin.implClass).select(getter.name).appliedTo(This(ctx.owner.asClass))
+                 else default
+               } else initial)
+          )
         else if (isScala2x) EmptyTree
         else initial
       }