dynamic type association raise warning when using null-safety

[Jictyvoo]

When doing an operation like

var a = 'foo';
final b = <String, dynamic>{'bar' : 'foo-bar'};
a = b['bar'];

This operation should raise a null warning or error. Sometimes, when this map can be outside the function, the programmer can know received types, but as a dynamic type the linter should show a warning or error about null when using null-safety.

Examples

class A {
    final String field;
    A.fromJson(Map<String, dynamic> json):  field = json['foo-bar'];
}

In above example, you can't know if field decoded in json is null or not.
Maybe can be a new rule that can be added in analysis_options.yaml

[srawlins]

We have a 'strict-casts' mode that reports the warnings you want. https://dart.dev/tools/analysis#enabling-additional-type-checks