Prepare to enable import/prefer-default-export

This was recently added to eslint-plugin-export. It enforces that
modules that only have a single export use a default export instead of a
named export. Since this is a breaking change and we want to cluster
breaking changes, I marked it as 0 for now with a TODO to enable when
that time comes.

README.md

@@ -1153,6 +1153,18 @@ Other Style Guides
     export { foo }
     ```
 
+  <a name="modules--prefer-default-export"></a>
+  - [10.6](#modules--prefer-default-export) In modules with a single export, prefer default export over named export.
+ eslint: [`import/prefer-default-export`](https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/prefer-default-export.md)
+
+    ```javascript
+    // bad
+    export function foo() {}
+
+    // good
+    export default function foo() {}
+    ```
+
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 ## Iterators and Generators

packages/eslint-config-airbnb-base/rules/es6.js

@@ -203,6 +203,11 @@ module.exports = {
       'newlines-between': 'never',
     }],
 
+    // Require modules with a single export to use a default export
+    // https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/prefer-default-export.md
+    // TODO: enable
+    'import/prefer-default-export': 0,
+
     // Require a newline after the last import/require in a group
     // https://github.com/benmosher/eslint-plugin-import/blob/master/docs/rules/newline-after-import.md
     // TODO: enable