Custom Cookie

[negator]

Implements a custom dart cookie implementation to supersede the existing one from dart:io package. The reasons for this are to due with RFC compliance, performance and flexibility.

• The dart:io implementation is overly strict on the cookie names it accepts, namely it errors out if certain characters are included in the name: https://github.com/dart-lang/sdk/blob/d63d5d93a59b370bf5f5c277109110cb67a96b75/sdk/lib/_http/http.dart#L989-L995 This restriction is not enforced by web browsers, for example you can include the @ symbol in a cookie name without error.
• The dart:io implementation performs unnecessary validation and parsing. Since cookies originate direcrtly from webview cookie managers, validation is completely unnecessary and wasteful.
• The dart:io implementation does not have support for JSON serialization.

This patch additionally makes updates to the existing setCookies and clearCookies functions. Namely, the functions completely finish their operations (setting or clearing) before returning an result asynchronously. Prior to this patch, the functions returned immediately, without waiting for completion leading to race conditions.