8325189: Enable this-escape javac warning in java.base

Reviewed-by: alanb, erikj, naoto, smarks, ihse, joehw, lancea, weijun

src/java.base/share/classes/java/util/Hashtable.java

@@ -226,6 +226,7 @@ public class Hashtable<K,V>
      * @throws NullPointerException if the specified map is null.
      * @since   1.2
      */
+    @SuppressWarnings("this-escape")
     public Hashtable(Map<? extends K, ? extends V> t) {
         this(Math.max(2*t.size(), 11), 0.75f);
         putAll(t);