title: Typescript compilation error: <x> is not assignable to type <y>
body: <p>When I am trying to assign </p>
  
  <pre><code>let blob:Blob = {blobParts: [content]};
  </code></pre>
  
  <p>where content is a json.</p>
  
  <p>where <code>Blob</code> is declared in <code>lib.d.ts</code> as </p>
  
  <pre><code>  interface Blob {
        readonly size: number;
        readonly type: string;
        msClose(): void;
        msDetachStream(): any;
        slice(start?: number, end?: number, contentType?: string): Blob;
    }
  
    declare var Blob: {
      prototype: Blob;
      new (blobParts?: any[], options?: BlobPropertyBag): Blob;
    };
  </code></pre>
  
  <p>it is giving me the following error:</p>
  
  <pre><code>Type '{ blobParts: any[]; }' is not assignable to type 'Blob'. Object literal may only specify known properties, and 'blobParts' does not exist in type 'Blob'.
  </code></pre>
  
  <p>I have verified the spelling of blobParts a number of times</p>