Add typings for @storybook/addon-a11y (#23210)

types/storybook__addon-a11y/index.d.ts

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+// Type definitions for @storybook/addon-a11y 3.3
+// Project: https://github.com/storybooks/storybook
+// Definitions by: HyunSeob <https://github.com/hyunseob>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.6
+
+import { StoryDecorator } from '@storybook/react';
+
+export const checkA11y: StoryDecorator;

types/storybook__addon-a11y/storybook__addon-a11y-tests.tsx

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+import * as React from 'react';
+import { storiesOf } from '@storybook/react';
+import { checkA11y } from '@storybook/addon-a11y';
+
+storiesOf('button', module)
+  .addDecorator(checkA11y)
+  .add('Accessible', () => (
+    <button>
+      Accessible button
+    </button>
+  ));

types/storybook__addon-a11y/tsconfig.json

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+{
+  "compilerOptions": {
+      "module": "commonjs",
+      "lib": [
+          "es6"
+      ],
+      "noImplicitAny": true,
+      "noImplicitThis": true,
+      "strictNullChecks": true,
+      "strictFunctionTypes": true,
+      "baseUrl": "../",
+      "jsx": "react",
+      "typeRoots": [
+          "../"
+      ],
+      "paths": {
+          "@storybook/addon-a11y": [
+              "storybook__addon-a11y"
+          ],
+          "@storybook/react": [
+              "storybook__react"
+          ]
+      },
+      "types": [],
+      "noEmit": true,
+      "forceConsistentCasingInFileNames": true
+  },
+  "files": [
+      "index.d.ts",
+      "storybook__addon-a11y-tests.tsx"
+  ]
+}

types/storybook__addon-a11y/tslint.json

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+{ "extends": "dtslint/dt.json" }