Remove unsafe implicit cast warnings in Flutter code base

[kasperl]

The Flutter analysis options currently contain:

strong_mode_down_cast_composite: ignore

Enabling those warnings leads to >100 warnings in the Flutter code base like these:

[warning] Unsafe implicit cast from 'Map<dynamic, dynamic>' to 'Map<String, dynamic>'. 
[warning] Unsafe implicit cast from '(ByTooltipMessage) → Finder' to '(SerializableFinder) → Finder'.
...

I suspect that a lot of these will require changes to the Flutter code base, but we should make sure that the type system isn't making this unnecessarily hard.

Reproduction steps:

   $ git clone https://github.com/flutter/flutter.git
   $ cd flutter
   $ bin/flutter update-packages
   <<comment out the strong_mode_down_cast_composite in .analysis_options_repo>>
   $ bin/flutter analyze --flutter-repo

[kasperl]

If these warnings are only needed because we don't do the correct runtime strong mode checks, we should think about what state we want this to be in for 1.23.

[kasperl]

We believe we can get rid of this warning and turn it into a lint that users can enable if they want help finding areas where runtime checks might fail for them.

[kasperl]

We have reached the decision to turn off the strong_mode_down_cast_composite warning by default, but we will do it in a few steps. First, we will turn it off by default in Flutter projects but keep it on for web projects.

It looks like analysis options are dealt with in a lot of places in the analyzer code base, @pq? It feels hard to manipulate the options map and add the ignore there, so here's a suggestion on how to turn this off for Flutter projects by passing down the hasFlutterDependency flag to the strong mode checker: https://codereview.chromium.org/2703253006/.

Do you have cycles to look at this, Phil, and figure out what the best path forward is?

[pq]

Do you have cycles to look at this, Phil, and figure out what the best path forward is?

👍 Happy to take a look.

[kasperl]

Turned the warning into a hint in fa2a3a1, but had to revert it.

[kasperl]

Finally landed turning this into a lint in 2fe8485.