Added typedef for @reach/alert (#35509)

types/reach__alert/index.d.ts

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+// Type definitions for @reach/alert 0.1
+// Project: https://github.com/reach/reach-ui
+// Definitions by: Harry Hedger <https://github.com/hedgerh>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 2.8
+
+import * as React from 'react';
+
+export type AlertProps = {
+    type?: "assertive" | "polite";
+} & React.HTMLProps<HTMLDivElement>;
+
+export const Alert: React.FC<AlertProps>;

types/reach__alert/reach__alert-tests.tsx

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+import { Alert } from '@reach/alert';
+
+import * as React from "react";
+import { render } from "react-dom";
+
+render(<Alert />, document.getElementById('app'));
+
+// should handle both types
+render(<Alert type="assertive" />, document.getElementById('app'));
+render(<Alert type="polite" />, document.getElementById('app'));
+
+// Should take div props and spread them down to div tag
+render(<Alert onDoubleClick={() => {}} />, document.getElementById('app'));
+render(
+    <Alert type="assertive">This is a standard alert.</Alert>,
+    document.getElementById('app')
+);
+
+// Should explode on a bad type
+// $ExpectError
+render(<Alert type="foo" />, document.getElementById('app'));
+
+// Should throw on a bad prop
+// $ExpectError
+render(<Alert badProp />, document.getElementById('app'));

types/reach__alert/tsconfig.json

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

types/reach__alert/tslint.json

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