prefer-stateless-function w/ decorators

Currently prefer-stateless-function warns when using a decorated class
over a stateless function. The decorator syntax only works with classes,
so it makes sense not to warn in this case.

Fixes #1034

Author: benstepp

docs/rules/prefer-stateless-function.md

@@ -10,6 +10,7 @@ This rule will check your class based React components for
 * instance property other than `this.props` and `this.context`
 * extension of `React.PureComponent` (if the `ignorePureComponents` flag is true)
 * presence of `ref` attribute in JSX
+* the use of decorators
 * `render` method that return anything but JSX: `undefined`, `null`, etc. (only in React <15.0.0, see [shared settings](https://github.com/yannickcr/eslint-plugin-react/blob/master/README.md#configuration) for React version configuration)
 
 If none of these elements are found, the rule will warn you to write this component as a pure function.

lib/rules/prefer-stateless-function.js

@@ -286,11 +286,25 @@ module.exports = {
       });
     }
 
+    /**
+     * Mark a ClassDeclaration as having used decorators
+     * @param {ASTNode} node The AST node being checked.
+     */
+    function markDecoratorsAsUsed(node) {
+      components.set(node, {
+        useDecorators: true
+      });
+    }
+
     return {
       ClassDeclaration: function (node) {
         if (ignorePureComponents && utils.isPureComponent(node)) {
           markSCUAsDeclared(node);
         }
+
+        if (node.decorators && node.decorators.length) {
+          markDecoratorsAsUsed(node);
+        }
       },
 
       // Mark `this` destructuring as a usage of `this`
@@ -378,6 +392,7 @@ module.exports = {
             list[component].useRef ||
             list[component].invalidReturn ||
             list[component].hasChildContextTypes ||
+            list[component].useDecorators ||
             (!utils.isES5Component(list[component].node) && !utils.isES6Component(list[component].node))
           ) {
             continue;

tests/lib/rules/prefer-stateless-function.js

@@ -268,6 +268,40 @@ ruleTester.run('prefer-stateless-function', rule, {
         '};'
       ].join('\n'),
       parser: 'babel-eslint'
+    }, {
+      // Uses a decorator
+      code: [
+        '@foo',
+        'class Foo extends React.Component {',
+        '  render() {',
+        '    return <div>{this.props.foo}</div>;',
+        '  }',
+        '}'
+      ].join('\n'),
+      parser: 'babel-eslint'
+    }, {
+      // Uses a called decorator
+      code: [
+        '@foo("bar")',
+        'class Foo extends React.Component {',
+        '  render() {',
+        '    return <div>{this.props.foo}</div>;',
+        '  }',
+        '}'
+      ].join('\n'),
+      parser: 'babel-eslint'
+    }, {
+      // Uses multiple decorators
+      code: [
+        '@foo',
+        '@bar()',
+        'class Foo extends React.Component {',
+        '  render() {',
+        '    return <div>{this.props.foo}</div>;',
+        '  }',
+        '}'
+      ].join('\n'),
+      parser: 'babel-eslint'
     }
   ],