Remove the check for console.warn

We already polyfill this at FB and we should recommend https://github.com/paulmillr/console-polyfill for open source.

src/core/ReactComponent.js

@@ -106,7 +106,7 @@ function validateExplicitKey(component) {
     message += ' It was passed a child from ' + childOwnerName + '.';
   }
 
-  global.console && console.warn && console.warn(message);
+  console.warn(message);
 }
 
 /**

src/dom/components/ReactDOMTextarea.js

@@ -53,7 +53,7 @@ var ReactDOMTextarea = ReactCompositeComponent.createClass({
     // TODO (yungsters): Remove support for children content in <textarea>.
     var children = this.props.children;
     if (children != null) {
-      global.console && console.warn && console.warn(
+      console.warn(
         'Use the `defaultValue` or `value` props instead of setting children ' +
         'on <textarea>.'
       );