Clone on mount

This is the first step towards descriptors. This will start cloning the
component when it's mounted instead of mounting the first instance.

This avoids an issue where a reference to the first instance can hang around
in props. Since a mounted component gets mutated, the descriptor changes.

We don't need to clone the props object itself. Mutating the shallow props
object of a child that's passed into you is already flawed. Those cases need to
use cloneWithProps. A props object is considered shallow frozen after it leaves
the render it was created in.

src/browser/ReactTextComponent.js

@@ -45,6 +45,16 @@ var ReactTextComponent = function(initialText) {
   this.construct({text: initialText});
 };
 
+/**
+ * Used to clone the text descriptor object before it's mounted.
+ *
+ * @param {object} props
+ * @return {object} A new ReactTextComponent instance
+ */
+ReactTextComponent.ConvenienceConstructor = function(props) {
+  return new ReactTextComponent(props.text);
+};
+
 mixInto(ReactTextComponent, ReactComponent.Mixin);
 mixInto(ReactTextComponent, ReactBrowserComponentMixin);
 mixInto(ReactTextComponent, {