Don't lower case HTML tags in comparison for built-ins (facebook#21155)

Author: sebmarkbage

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

@@ -1221,7 +1221,7 @@ describe('ReactDOMComponent', () => {
       }
     });
 
-    it('should not duplicate uppercased selfclosing tags', () => {
+    it('should warn for uppercased selfclosing tags', () => {
       class Container extends React.Component {
         render() {
           return React.createElement('BR', null);
@@ -1237,7 +1237,8 @@ describe('ReactDOMComponent', () => {
           'Use PascalCase for React components, ' +
           'or lowercase for HTML elements.',
       );
-      expect(returnedValue).not.toContain('</BR>');
+      // This includes a duplicate tag because we didn't treat this as self-closing.
+      expect(returnedValue).toContain('</BR>');
     });
 
     it('should warn on upper case HTML tags, not SVG nor custom tags', () => {

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

@@ -1324,7 +1324,7 @@ class ReactDOMServerRenderer {
     context: Object,
     parentNamespace: string,
   ): string {
-    const tag = element.type.toLowerCase();
+    const tag = element.type;
 
     let namespace = parentNamespace;
     if (parentNamespace === HTML_NAMESPACE) {
@@ -1335,7 +1335,7 @@ class ReactDOMServerRenderer {
       if (namespace === HTML_NAMESPACE) {
         // Should this check be gated by parent namespace? Not sure we want to
         // allow <SVG> or <mATH>.
-        if (tag !== element.type) {
+        if (tag.toLowerCase() !== element.type) {
           console.error(
             '<%s /> is using incorrect casing. ' +
               'Use PascalCase for React components, ' +