title: Fail to create variable using eval in Node.js ES6
body: <p>It seems not possible to create a variable using <code>eval</code> in Node.js ES6 but I can't understand why. This happens to me on CentOS 7, but I don't believe OS is the problem here.</p>
  
  <p>Regular Node.js file (test.js):</p>
  
  <pre><code>eval("var a=1");
  console.log(a);
  </code></pre>
  
  <p>Make the same file with .mjs extension to run with Node.js ES6 (test.mjs):</p>
  
  <pre><code>eval("var a=1");
  console.log(a);
  </code></pre>
  
  <p>After that, run the 2 files with Node.js, and Node.js ES6:</p>
  
  <pre><code>$ node test.js
  1
  
  $ node --experimental-modules test.mjs
  (node:9966) ExperimentalWarning: The ESM module loader is experimental.
  ReferenceError: a is not defined
      at file:///temp/test.mjs:2:13
      at ModuleJob.run (internal/modules/esm/module_job.js:96:12)
  </code></pre>
  
  <p>Is it an issue related to ES6? I tried on browser's console and the problem is the same:</p>
  
  <pre><code>&gt;&gt; eval("var a=1"); console.log(a);
     1
  
  &gt;&gt; class c { static f(){ eval("var a=1"); console.log(a); } }
     c.f()
     ReferenceError: a is not defined
  </code></pre>
  
  <p>I'm using Node.js 10.9.0, is it a bug or there's a reason behind it?</p>