DeCo. Centralize resolver scope helpers in ScopeContext.

Expand ScopeContext into the single place that manages resolver
name-scope transitions, and migrate visitors to use it directly.

ScopeContext now:
- Owns the current name scope and handles push/pop via withScope().
- Provides focused helpers for common resolver scopes:
  - withLocalScope(), withInstanceScope(), withExtensionScope()
  - withTypeParameterScope() and withTypeParameterList()
  - withFormalParameterScope()
  - withConstructorInitializerScope() and withPrimaryParameterScope()
  - withDocImportScope() for documentation comment resolution
- Renames walkMixinDeclarationScopes() to visitMixinDeclaration() and routes
  mixin traversal through the shared helper.

Update ResolutionVisitor, ScopeResolverVisitor, and ReferenceResolver

to:
- Remove temporary routing getters/methods and ad-hoc try/finally scope
  management.
- Use the new ScopeContext helpers for consistent scoping across declarations,
  function bodies, and DeCo/primary-constructor-related initializer contexts.
- Reduce duplicated scope wiring and keep scope behavior localized for easier
  future evolution.

This is reland of https://dart-review.googlesource.com/c/sdk/+/481981 with support for ScopeResolverVisitor.visitAnonymousMethodInvocation, see PS(s).

Change-Id: Ieee6c732872fc32af86710237972cd7397ca54dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/482365
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>