[nomerge] Export underlying on non-copying conversion

Any `TraversableForwarder` method with the danger of returning
`underlying` itself needs to go through `toList` (which marks the
underlying list as needing to be copied on the next mutation).

Notice that `toStream` is also affected since `List#toStream` builds the
result lazily.

Fixes scala/bug#11869

Author: hrhino

library/src/scala/collection/mutable/ListBuffer.scala

@@ -306,13 +306,18 @@ final class ListBuffer[A]
   def result: List[A] = toList
 
   /** Converts this buffer to a list. Takes constant time. The buffer is
-   *  copied lazily, the first time it is mutated.
+   *  copied lazily the first time it is mutated.
    */
   override def toList: List[A] = {
     exported = !isEmpty
     start
   }
 
+  // scala/bug#11869
+  override def toSeq: collection.Seq[A] = toList
+  override def toIterable: collection.Iterable[A] = toList
+  override def toStream: immutable.Stream[A] = toList.toStream // mind the laziness
+
 // New methods in ListBuffer
 
   /** Prepends the elements of this buffer to a given list