[tests] Primary constructors - Tests on assignment, naming conflicts, and more.

Language tests for the following:
- A compile-time error occurs if an assignment to a primary parameter
occurs in the initializer list of the body part of a primary
constructor.
- A compile-time error occurs if an instance variable declaration has an
initializing expression, and it is also initialized by an element in the
initializer list of the body part, or by an initializing formal of the
primary constructor.
- The modifier `final` on a parameter in a declaring constructor
specifies that the instance variable declaration which is induced by
this declaring constructor parameter is `final`.
- A compile-time error occurs if a class, enum, or extension type has a
primary constructor whose name is also the name of a constructor
declared in the body.
- Extension types are allowed to have non-redirecting generative
constructors, implicitly or explicitly initializing the representation
variable.
- A compile-time error occurs if a class, mixin class, enum, or
extension type declares a primary constructor whose name is `C.n`, and
the body declares a static member whose basename is `n`.
- Consider a factory constructor declaration of the form `factory C(...`
optionally starting with the modifier `const`. Assume that `C` is the
name of the enclosing class, mixin class, enum, or extension type. In
this situation, the declaration declares a constructor whose name is
`C`. The rule which is being tested here is also applicable when the
declaration has some of the keywords external and augment, but we don't
test those kinds of constructors here.
- It is an error to have multiple primary constructor bodies.
- Tests for type inference of declaring parameters in primary
constructors, including override inference and inference from default
values.

Bug: https://github.com/dart-lang/sdk/issues/61687
Change-Id: If804c99c317ab53643420e00d6dc266720bca2f1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/483626
Commit-Queue: Kallen Tu <kallentu@google.com>
Reviewed-by: Erik Ernst <eernst@google.com>