Accept nested Map in Uri constructor

[FranklinYu]

In current Uri constructor, parameter queryParameters accepts a Map with value type String or Iterable<String>. It targets two scenario:

foo=1&bar=true
foo=1&foo=2

However, many backends also support nested structures like

name[first]=Tom&name[last]=Hanks&gender=male

which will be translated (in backend) to

{
  "name": {
    "first": "Tom",
    "last": "Hanks"
  },
  "gender": "male"
}

It would be really handy if we can just pass in

Uri(queryParameters: { 'name': {'first': 'Tom', 'last': 'Hanks'}, 'gender': 'male'})

especially when the Map<String, dynamic> is constructed from somewhere else.

[lrhn]

It's doable, but I'm not sure it's recommendable.

The [ and ] characters are general delimiters in URI syntax, so they need to be escaped, even in the query part. That, by itself, isn't a problem, since building the query from the map can just use the escaped characters directly.
You probably also want to be able to parse them back, which means that parsing query parameters gets more expensive. Currently we can split on & and = and not worry about escaped characters when we build the query parameters mapping. Again, not necessarily a problem, we can probably detect an "indexed" name just by seeing that it ends in (escaped) ].
We need to be able to handle cases like ?foo=baz&foo[bar]=qux in some way (probably just ['baz', {'bar': 'qux'}], unless there is some other preferred way of doing it).

So, extra overhead, but probably not much if you don't use it, and I can see the usefulness.