Fix IE11 placeholder textContent value bug. (#7002)

src/renderers/dom/client/wrappers/ReactDOMTextarea.js

@@ -187,7 +187,9 @@ var ReactDOMTextarea = {
     // This is in postMount because we need access to the DOM node, which is not
     // available until after the component has mounted.
     var node = ReactDOMComponentTree.getNodeFromInstance(inst);
-    node.value = node.value; // Detach value from defaultValue
+
+    // Warning: node.value may be the empty string at this point (IE11) if placeholder is set.
+    node.value = node.textContent; // Detach value from defaultValue
   },
 };