Make findDOMNode error clearer

Fixes facebook#4233.

Author: sophiebits

src/renderers/dom/client/__tests__/findDOMNode-test.js

@@ -44,12 +44,19 @@ describe('findDOMNode', function() {
   });
 
   it('findDOMNode should reject unmounted objects with render func', function() {
-    expect(function() {
-      ReactDOM.findDOMNode({render: function() {}});
-    })
-      .toThrow('Invariant Violation: Component (with keys: render) ' +
-        'contains `render` method but is not mounted in the DOM'
-      );
+    var Foo = React.createClass({
+      render: function() {
+        return <div />;
+      },
+    });
+
+    var container = document.createElement('div');
+    var inst = ReactDOM.render(<Foo />, container);
+    ReactDOM.unmountComponentAtNode(container);
+
+    expect(() => ReactDOM.findDOMNode(inst)).toThrow(
+      'Invariant Violation: findDOMNode was called on an unmounted component.'
+    );
   });
 
 });

src/renderers/dom/client/findDOMNode.js

@@ -53,9 +53,7 @@ function findDOMNode(componentOrElement) {
   invariant(
     componentOrElement.render == null ||
     typeof componentOrElement.render !== 'function',
-    'Component (with keys: %s) contains `render` method ' +
-    'but is not mounted in the DOM',
-    Object.keys(componentOrElement)
+    'findDOMNode was called on an unmounted component.'
   );
   invariant(
     false,