Find callsites that would call toString() if we pass attributes throu…

…gh (#10416)

* Find callsites that would call toString() if we pass attributes through

* Warn for all object unknown props, not just 'toString' ones

fixing this as per Sebastian's feedback, because we want to find all
objects and not just ones with 'toString' defined. Because there may be
other corner cases and issues that are revealed.

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

@@ -126,6 +126,18 @@ var warnUnknownProperties = function(type, props, debugID) {
     var isValid = validateProperty(type, key, debugID);
     if (!isValid) {
       unknownProps.push(key);
+      var value = props[key];
+      if (typeof value === 'object' && value !== null) {
+        warning(
+          false,
+          'The %s prop on <%s> is not a known property, and was given an object.' +
+            'Remove it, or it will appear in the ' +
+            'DOM after a future React update.%s',
+          key,
+          type,
+          getStackAddendum(debugID),
+        );
+      }
     }
   }