Minor: context/theme-detailed-app.js broken (#746)

* Minor: context/theme-detailed-app.js broken

Today I tested out the new Context API using using [CRA](https://github.com/facebook/create-react-app). 
I encountered a runtime exception, so this tiny PR is intended to make it easier for future Context API learners.

`ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from derived constructor`

create-react-app@next commit da518d2238298dd5db74e9c63c12251099f87bb0

* Update theme-detailed-app.js

examples/context/theme-detailed-app.js

@@ -12,6 +12,7 @@ function Toolbar(props) {
 
 class App extends React.Component {
   constructor(props) {
+    super(props);
     this.state = {
       theme: themes.light,
     };