Limit the meaning of "custom element" to not include is

Effectively this means that you can't use custom properties/events, other
than the ones React knows about on `is` extensions.

This is unfortunate but there's too many paths that are forked in
inconsistent ways. I think the solution is to let all React elements set
unknown properties in the same way as this flag but that's a bigger change
than this flag implies.

Since `is` is not universally supported yet anyway, this doesn't seem like
a huge loss.

Attributes still work.

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

@@ -2604,7 +2604,7 @@ export function pushStartInstance(
       );
     }
     default: {
-      if (type.indexOf('-') === -1 && typeof props.is !== 'string') {
+      if (type.indexOf('-') === -1) {
         // Generic element
         return pushStartGenericElement(target, props, type);
       } else {

packages/react-dom-bindings/src/shared/ReactDOMUnknownPropertyHook.js

@@ -308,7 +308,7 @@ function warnUnknownProperties(type, props, eventRegistry) {
 }
 
 export function validateProperties(type, props, eventRegistry) {
-  if (isCustomElement(type, props)) {
+  if (isCustomElement(type, props) || typeof props.is === 'string') {
     return;
   }
   warnUnknownProperties(type, props, eventRegistry);

packages/react-dom-bindings/src/shared/isCustomElement.js

@@ -9,7 +9,7 @@
 
 function isCustomElement(tagName: string, props: Object): boolean {
   if (tagName.indexOf('-') === -1) {
-    return typeof props.is === 'string';
+    return false;
   }
   switch (tagName) {
     // These are reserved SVG and MathML elements.

packages/react-dom/src/__tests__/ReactDOMServerIntegrationAttributes-test.js

@@ -669,8 +669,14 @@ describe('ReactDOMServerIntegration', () => {
       }
     });
 
-    itRenders('htmlFor attribute on custom elements', async render => {
+    itRenders('htmlFor property on is elements', async render => {
       const e = await render(<div is="custom-element" htmlFor="test" />);
+      expect(e.getAttribute('htmlFor')).toBe(null);
+      expect(e.getAttribute('for')).toBe('test');
+    });
+
+    itRenders('htmlFor attribute on custom elements', async render => {
+      const e = await render(<custom-element htmlFor="test" />);
       expect(e.getAttribute('htmlFor')).toBe('test');
       expect(e.getAttribute('for')).toBe(null);
     });