Deduplicate "unknown prop" warning to ease pain of version updates

src/renderers/dom/shared/__tests__/ReactDOMComponent-test.js

@@ -172,6 +172,21 @@ describe('ReactDOMComponent', () => {
       );
     });
 
+    it('should only warn for the first unknown props passed', () => {
+      spyOn(console, 'error');
+      var container = document.createElement('div');
+      ReactDOM.render(
+        // the 'span' should not trigger a second warning in React v15.6.0
+        <div foo="bar" baz="qux"><span hello="world" /></div>,
+        container,
+      );
+      expect(console.error.calls.count(0)).toBe(1);
+      expect(normalizeCodeLocInfo(console.error.calls.argsFor(0)[0])).toBe(
+        'Warning: Unknown props `foo`, `baz` on <div> tag. Remove these props from the element. ' +
+        'For details, see https://fb.me/react-unknown-prop\n    in div (at **)'
+      );
+    });
+
     it('should warn for onDblClick prop', () => {
       spyOn(console, 'error');
       var container = document.createElement('div');

src/renderers/dom/shared/hooks/ReactDOMUnknownPropertyHook.js

@@ -95,7 +95,13 @@ if (__DEV__) {
   };
 }
 
+let alreadyWarned = false;
+
 var warnUnknownProperties = function(debugID, element) {
+  if (alreadyWarned) {
+    return;
+  }
+
   var unknownProps = [];
   for (var key in element.props) {
     var isValid = validateProperty(element.type, key, debugID);
@@ -117,6 +123,7 @@ var warnUnknownProperties = function(debugID, element) {
       element.type,
       ReactComponentTreeHook.getStackAddendumByID(debugID)
     );
+    alreadyWarned = true;
   } else if (unknownProps.length > 1) {
     warning(
       false,
@@ -126,6 +133,7 @@ var warnUnknownProperties = function(debugID, element) {
       element.type,
       ReactComponentTreeHook.getStackAddendumByID(debugID)
     );
+    alreadyWarned = true;
   }
 };