title: Angular - Cannot add property xxxx, object is not extensible
body: <p>I Use Typescript (Angular) And I have this error: <code>Cannot add property media, object is not extensible</code></p>
  
  <p>I want add a Media element in My step Object. At the origine Step object do not contain any Media array.</p>
  
  <pre><code>export interface Step {
    duration: string;
    instructions: string;
    ingredients_instructions?: string;
    ingredients?: Quantity[];
    media?: Media[];
    action?: CookingAction;
  }
  
  export interface Media {
    id: string;
    author: string;
    license: string;
    creation_date: string;
    modification_date: string;
    bytes: string;
  }
  
  stepsSelection: Step[] = [];
  ...
  const media = {} as Media;
  media.bytes = 'foo';
  media.license = 'DR';
  this.stepsSelection[i].media = [ ];
  this.stepsSelection[i].media[0] = media;
  </code></pre>