Add ConcurrentHashMap.newKeySet()

PiperOrigin-RevId: 369785291
Change-Id: I5af817f6645149e08ea7fec37977f3a5aabf074b

user/super/com/google/gwt/emul/java/util/concurrent/ConcurrentHashMap.java

diff --git a/user/super/com/google/gwt/emul/java/util/concurrent/ConcurrentHashMap.java b/user/super/com/google/gwt/emul/java/util/concurrent/ConcurrentHashMap.java
index d8a0371..8f4ff38 100644
--- a/user/super/com/google/gwt/emul/java/util/concurrent/ConcurrentHashMap.java
+++ b/user/super/com/google/gwt/emul/java/util/concurrent/ConcurrentHashMap.java
@@ -22,6 +22,7 @@
 import java.util.Collections;
 import java.util.Enumeration;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
 
@@ -142,4 +143,8 @@
   public Enumeration<K> keys() {
     return Collections.enumeration(keySet());
   }
+
+  public static <T> Set<T> newKeySet() {
+    return new HashSet<>();
+  }
 }