DOM FileReader load event needs to be more specific

[prettydiff]

Looking at lib.dom.d.ts the FileReader event is:

interface FileReader extends EventTarget, MSBaseReader {
    readonly error: DOMError;
    readAsArrayBuffer(blob: Blob): void;
    readAsBinaryString(blob: Blob): void;
    readAsDataURL(blob: Blob): void;
    readAsText(blob: Blob, encoding?: string): void;
    addEventListener<K extends keyof MSBaseReaderEventMap>(type: K, listener: (this: FileReader, ev: MSBaseReaderEventMap[K]) => any, useCapture?: boolean): void;
    addEventListener(type: string, listener: EventListenerOrEventListenerObject, useCapture?: boolean): void;
    removeEventListener<K extends keyof MSBaseReaderEventMap>(type: K, listener: (this: FileReader, ev: MSBaseReaderEventMap[K]) => any, useCapture?: boolean): void;
    removeEventListener(type: string, listener: EventListenerOrEventListenerObject, useCapture?: boolean): void;
}

declare var FileReader: {
    prototype: FileReader;
    new(): FileReader;
};

Its events are defined as MSBaseReaderEventMap:

interface MSBaseReaderEventMap {
    "abort": Event;
    "error": ErrorEvent;
    "load": Event;
    "loadend": ProgressEvent;
    "loadstart": Event;
    "progress": ProgressEvent;
}

The load event is of generic event type Event, but that will not work for the FileReader object. The load event contains the property event.target as do all events generically, but it does not contain the property event.target.result, which is where the FileReader's data will reside.

https://developer.mozilla.org/en-US/docs/Web/API/FileReader/onload

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

The lib files in this repo are autogenerated by a script in https://github.com/Microsoft/TSJS-lib-generator. changes like these need to be made to the generator instead.