Add JsOptional annotation.
Change-Id: I09cfd5d0825d4b148dd3410f052eb0e147e8769b
diff --git a/user/src/jsinterop/annotations/JsOptional.java b/user/src/jsinterop/annotations/JsOptional.java
new file mode 100644
index 0000000..5ff5f49
--- /dev/null
+++ b/user/src/jsinterop/annotations/JsOptional.java
@@ -0,0 +1,39 @@
+/*
+ * Copyright 2016 Google Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package jsinterop.annotations;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * JsOptional marks a parameter in a method as optional indicating that the argument can be omitted
+ * from the function call when called from JavaScript side.
+ *
+ * <p>Note that JsOptional can only be used in a JsConstructor, a JsMethod or a JsFunction method.
+ * An optional argument cannot precede a non-optional argument (a vararg is considered an
+ * optional argument).
+ *
+ * <p>This annotation is informational in the GWT compiler but other compilers or tools might use it
+ * for example to annotate types in the output JavaScript program.
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target(ElementType.PARAMETER)
+@Documented
+public @interface JsOptional {
+}
