Make Collectors non-instantiatable

Change-Id: I3c750b05a1582b1be17dff3edad2166a8f0ccbaa

user/super/com/google/gwt/emul/java/util/stream/Collectors.java

diff --git a/user/super/com/google/gwt/emul/java/util/stream/Collectors.java b/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
index cb67745..43930ec 100644
--- a/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
+++ b/user/super/com/google/gwt/emul/java/util/stream/Collectors.java
@@ -396,4 +396,6 @@
     collection.addAll(items);
     return collection;
   }
+
+  private Collectors() { }
 }