Add `Optional.isEmpty()` to GWT emulation.

PiperOrigin-RevId: 399696516
Change-Id: I7477a72cf4e5abd954c8e159188863c5d4d32100

user/super/com/google/gwt/emul/java/util/Optional.java

diff --git a/user/super/com/google/gwt/emul/java/util/Optional.java b/user/super/com/google/gwt/emul/java/util/Optional.java
index 8dc4a8f..3258444 100644
--- a/user/super/com/google/gwt/emul/java/util/Optional.java
+++ b/user/super/com/google/gwt/emul/java/util/Optional.java
@@ -57,6 +57,10 @@
     return ref != null;
   }
 
+  public boolean isEmpty() {
+    return ref == null;
+  }
+
   public T get() {
     checkCriticalElement(isPresent());
     return ref;

user/test/com/google/gwt/emultest/java8/util/OptionalTest.java

diff --git a/user/test/com/google/gwt/emultest/java8/util/OptionalTest.java b/user/test/com/google/gwt/emultest/java8/util/OptionalTest.java
index ad5fa51..6790703 100644
--- a/user/test/com/google/gwt/emultest/java8/util/OptionalTest.java
+++ b/user/test/com/google/gwt/emultest/java8/util/OptionalTest.java
@@ -59,6 +59,21 @@
     assertTrue(present.isPresent());
   }
 
+  // Disabled until Java 11 is enabled in Open source
+/*  public void testIsEmpty() {
+    // empty case
+    assertTrue(empty.isEmpty());
+
+    empty = Optional.ofNullable(null);
+    assertTrue(empty.isEmpty());
+
+    // non-empty case
+    assertFalse(present.isEmpty());
+
+    present = Optional.ofNullable(REFERENCE);
+    assertFalse(present.isEmpty());
+  }*/
+
   public void testGet() {
     // empty case
     try {