feat: add fixer for requireValidFileAnnotation when always (#332)

* Add fixer for requireValidFileAnnotation when always

* Add autofix note in readme

Author: KevinGrandon

Diff for: README.md

@@ -2437,6 +2437,8 @@ This rule validates Flow file annotations.
 
 This rule can optionally report missing or missed placed annotations, common typos (e.g. `// @floww`), and enforce a consistant annotation style.
 
+This rule is autofixable with the `--fix` argument. This will autofix files by adding missing flow annotations to the top of each file. To avoid autofixing this rule per-file, you can add a `// @noflow` annotation to the top of individual files.
+
 <a name="eslint-plugin-flowtype-rules-require-valid-file-annotation-options"></a>
 #### Options
 

Diff for: src/rules/requireValidFileAnnotation.js

@@ -49,6 +49,14 @@ const create = (context) => {
     Program (node) {
       const firstToken = node.tokens[0];
 
+      const addAnnotation = () => {
+        return (fixer) => {
+          const annotation = ['line', 'none'].includes(style) ? '// @flow\n' : '/* @flow */\n';
+
+          return fixer.replaceTextRange([node.start, node.start], annotation);
+        };
+      };
+
       const potentialFlowFileAnnotation = _.find(context.getAllComments(), (comment) => {
         return looksLikeFlowFileAnnotation(comment.value);
       });
@@ -70,7 +78,11 @@ const create = (context) => {
           context.report(potentialFlowFileAnnotation, 'Malformed Flow file annotation.');
         }
       } else if (always) {
-        context.report(node, 'Flow file annotation is missing.');
+        context.report({
+          fix: addAnnotation(),
+          message: 'Flow file annotation is missing.',
+          node
+        });
       }
     }
   };

Diff for: tests/rules/assertions/requireValidFileAnnotation.js

@@ -122,6 +122,33 @@ export default {
           annotationStyle: 'block'
         }
       ]
+    },
+    {
+      code: 'a;',
+      errors: [
+        {
+          message: 'Flow file annotation is missing.'
+        }
+      ],
+      options: [
+        'always'
+      ],
+      output: '// @flow\na;'
+    },
+    {
+      code: 'a;',
+      errors: [
+        {
+          message: 'Flow file annotation is missing.'
+        }
+      ],
+      options: [
+        'always',
+        {
+          annotationStyle: 'block'
+        }
+      ],
+      output: '/* @flow */\na;'
     }
   ],
   misconfigured: [