Ignore function and symbol values on custom-elements on the server (facebook#21157)

Author: sebmarkbage

packages/react-dom/src/server/DOMMarkupOperations.js

@@ -71,7 +71,12 @@ export function createMarkupForCustomAttribute(
   name: string,
   value: mixed,
 ): string {
-  if (!isAttributeNameSafe(name) || value == null) {
+  if (
+    !isAttributeNameSafe(name) ||
+    value == null ||
+    typeof value === 'function' ||
+    typeof value === 'symbol'
+  ) {
     return '';
   }
   return name + '=' + quoteAttributeValueForBrowser(value);

packages/react-dom/src/server/ReactDOMServerFormatConfig.js

@@ -1070,7 +1070,11 @@ function pushStartCustomElement(
           // Ignored. These are built-in to React on the client.
           break;
         default:
-          if (isAttributeNameSafe(propKey)) {
+          if (
+            isAttributeNameSafe(propKey) &&
+            typeof propValue !== 'function' &&
+            typeof propValue !== 'symbol'
+          ) {
             target.push(
               attributeSeparator,
               stringToChunk(propKey),