Add Collections#singletonList

This is a very dumb implementation that wastes space and time by
constructing a full-blown ArrayList as backend. However, it is
better to have a dumb implementation than none at all, and we can
always do something about the performance when, and if, that should
become necessary.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

classpath/java/util/Collections.java

@@ -659,4 +659,10 @@ public class Collections {
       return - cmp.compare(o1, o2);
     }
   }
+
+  public static <T> List<T> singletonList(T o) {
+    ArrayList<T> list = new ArrayList<T>(1);
+    list.add(o);
+    return new UnmodifiableList(list);
+  }
 }