1.0.0-dev.5

• BREAKING: Renamed RequestInit to FetchOptions.
  This is done to make emphasis, that fetch and Request have different
  (default) behaviors.
• Added Request.
• Added RequestDuplex. This adds support for request streaming in supported
  browsers.
• Added ReadableStreamSource. This allows you to create custom
  ReadableStreams, that is required for request streaming.
• Added ReadableStreamDefaultController.
• Added ReadableStreamSource class that helps you to create custom source
  for ReadableStreams. It can be created from Dart'sStream.
• Added RequestCache.
• Added RequestDestination.
• Added missing options to RequestMode and added docs.
• Added FetchOptions and RequestOptions with corresponding docs from MDN.
• Compatibility layer:
  • BREAKING: Renamed createRequestInit to createFetchOptions
  • Added createRequestOptions.
  • Added createReadableStream.
  • Added createReadableStreamSourceFromStream.
• Fixed dev dependencies versions, to allow running on Dart 2.19.