Infer generic type parameters based on the declaring type

cross reference : https://github.com/felangel/bloc/issues/560

currently in dart, we have to explicitly specify a type parameter for generic types, even when they can be inferred.
e.g. this is a valid class definition
```dart
class Bloc {}
class BlocBuilder,TState> {}

class TestBloc extends Bloc {}
class TestBlocBuilder extends BlocBuilder {}
```

but this isn't
```dart
class Bloc {}
class BlocBuilder> {}

class TestBloc extends Bloc {}
class TestBlocBuilder extends BlocBuilder {}
```
you get an error at BlocBuilder definition:
```
The name 'TState' isn't a type so it can't be used as a type argument.
Try correcting the name to an existing type, or defining a type named 'TState'.
```

I am not sure if this is an intentional design choice, or a bug that no one noticed, but i sure hope this gets fixed.

Start by reading the cross-reference at felangel/bloc#560 and the examples in this issue, then investigate Dart's generic type-parameter inference and declaring-type rules. Done means reaching a language-design decision and specifying whether the omitted TState form is valid, including the resulting diagnostics and inference behavior.

Written by the indexing model from the issue text.