title: Property is missing in type '{ [x: string]: string; }'
body: <p>In this react app there is a form with a few input fields. These fields all use <code>this.handleChange</code> with the <code>onChange</code> attribute.    </p>
  
  <pre><code>private handleChange = (event: React.FormEvent&lt;HTMLInputElement&gt;) =&gt; {
      let target = event.target as HTMLInputElement;
      this.setState({
          [target.name]: target.value
      });
  };
  </code></pre>
  
  <p>The typescript error I get is:</p>
  
  <pre><code>ERROR in [at-loader] ./src/components/FormSubmitHighScore.tsx:43:23 
  TS2345: Argument of type '{ [x: string]: string; }' is not assignable to parameter of type 'Pick&lt;State, "first_name" | "last_name" | "email_address" | "school_name" | "region" | "submitted"&gt;'.
  Property 'first_name' is missing in type '{ [x: string]: string; }'.
  </code></pre>
  
  <p>So by setting the state name field dynamically (based on what input is being used) this Typescript error gets triggered (but it does work in the browser).</p>
  
  <p>How would I specify a correct type in this context?    </p>