Avoid .valueOf to close #20594 (#20617)

Author: dimaqq

packages/react-reconciler/src/ReactChildFiber.new.js

@@ -222,14 +222,15 @@ function coerceRef(
 
 function throwOnInvalidObjectType(returnFiber: Fiber, newChild: Object) {
   if (returnFiber.type !== 'textarea') {
+    const childString = Object.prototype.toString.call(newChild);
     invariant(
       false,
       'Objects are not valid as a React child (found: %s). ' +
         'If you meant to render a collection of children, use an array ' +
         'instead.',
-      Object.prototype.toString.call(newChild) === '[object Object]'
+      childString === '[object Object]'
         ? 'object with keys {' + Object.keys(newChild).join(', ') + '}'
-        : newChild,
+        : childString,
     );
   }
 }

packages/react-reconciler/src/ReactChildFiber.old.js

@@ -222,14 +222,15 @@ function coerceRef(
 
 function throwOnInvalidObjectType(returnFiber: Fiber, newChild: Object) {
   if (returnFiber.type !== 'textarea') {
+    const childString = Object.prototype.toString.call(newChild);
     invariant(
       false,
       'Objects are not valid as a React child (found: %s). ' +
         'If you meant to render a collection of children, use an array ' +
         'instead.',
-      Object.prototype.toString.call(newChild) === '[object Object]'
+      childString === '[object Object]'
         ? 'object with keys {' + Object.keys(newChild).join(', ') + '}'
-        : newChild,
+        : childString,
     );
   }
 }