Share the empty LinkedList between first0/last0.

This is in line with the existing implementation
of `MutableList#clear`, and in line with an newly
created empty collection.

Author: retronym

src/library/scala/collection/mutable/MutableList.scala

@@ -65,7 +65,7 @@ extends AbstractSeq[A]
     require(nonEmpty, "tail of empty list")
     tl.first0 = first0.tail
     tl.len = len - 1
-    tl.last0 = if (tl.len == 0) new LinkedList[A]() else last0
+    tl.last0 = if (tl.len == 0) tl.first0 else last0
   }
 
   /** Prepends a single element to this list. This operation takes constant

src/library/scala/collection/mutable/Queue.scala

@@ -183,7 +183,7 @@ extends MutableList[A]
 
   private[this] def decrementLength() {
     len -= 1
-    if (len == 0) last0 = new LinkedList[A]()
+    if (len == 0) last0 = first0
   }
 }