title: eslint-plugin-flowtype does not validate
body: <p>I am trying to configure <code>eslint + babel-eslint + eslint-plugin-react + eslint-plugin-flowtype</code></p>
  
  <p>I have the following <code>devDependencies</code> in <code>package.json</code>:</p>
  
  <pre><code>"babel-eslint": "^7.1.1",
  "eslint": "^3.10.2",
  "eslint-config-airbnb": "^13.0.0",
  "eslint-plugin-flowtype": "^2.25.0",
  "eslint-plugin-import": "^2.2.0",
  "eslint-plugin-jsx-a11y": "^2.2.3",
  "eslint-plugin-react": "^6.7.1"
  </code></pre>
  
  <p>And the following <code>.eslinrc</code>:</p>
  
  <pre><code>{
    "parser": "babel-eslint",
    "plugins": [
      "flowtype"
    ],
    "extends": [
      "airbnb",
      "plugin:flowtype/recommended"
    ],
    "rules": {
      "max-len": [1, 80, 2, {"ignoreComments": true}],
      "prop-types": [0],
      "react/jsx-filename-extension": [1, { "extensions": [".js", ".jsx"] }],
      "react/prefer-stateless-function": [
        0,
        {
          "ignorePureComponents": true
        }
      ],
      "no-console": 0
    },
    "settings": {
      "flowtype": {
        "onlyFilesWithFlowAnnotation": true
      }
    }
  }
  </code></pre>
  
  <p>I wrote simple code example in <code>App.js</code>:</p>
  
  <pre><code>function foo() {
    const a: string = 1;
    return a;
  }
  
  async function bar() {
    const b = await foo();
    return b;
  }
  </code></pre>
  
  <p>If I launch <code>eslint src/App.js</code> then <code>eslint</code> does not show any messages.
  If I add <code>"flowtype/require-return-type": 2</code> into <code>.eslintrc</code> then <code>eslint</code> shows:</p>
  
  <pre><code>error  Missing return type annotation  flowtype/require-return-type
  error  Missing return type annotation  flowtype/require-return-type
  ✖ 2 problems (2 errors, 0 warnings)
  </code></pre>
  
  <p>But I don't understand why <code>const a: string = 1;</code> is valid.
  How to enable checking type of <code>const a: string = 1;</code>?</p>