title: 'number' is never reassigned. Use 'const' instead. (prefer-const)
body: <p>Why in this case eslint 4.17.0 i have error number is never reassigned. Use 'const' instead. (prefer-const). Why i need to use const? Please, explain me i can't understand.</p>
  
  <pre><code>let test = {
      'number': 1,
      'string': 'asd',
  };
  test.number = 99;
  
  console.log(test.number);
  // output: 99
  </code></pre>
  
  <p>ecmascript</p>
  
  <pre><code> {
      "parser": "babel-eslint",
      "env": {
          "browser": true
      },
      "extends": [
          "google"
      ],
      "rules": {
          "prefer-const": 2
  
      },
      "parserOptions": {
          "ecmaVersion": 6,
          "sourceType": "module"
      }
  }
  </code></pre>
  
  <p>eslint problem</p>
  
  <pre><code>[eslint] 'test' is never reassigned. Use 'const' insted. (prefer-const)
  </code></pre>