[vm] Build scopes for implicit getters of static const fields

Usually implicit getters are not used for static const fields.
However, coverage can force their compilation. In order to compile
implicit getter of a static const field, its initializer is
evaluated in a constant evaluator. Initializer may have Let
expressions and can declare local variables.

Before this fix scope builder was not visiting initializer body for
implicit getters, which caused crashes in constant evaluator.

Fixes https://github.com/dart-lang/sdk/issues/35808

Change-Id: I807aeaa90886d84169c27275abdd0cfd0f3ff777
Reviewed-on: https://dart-review.googlesource.com/c/91705
Commit-Queue: Alexander Markov <alexmarkov@google.com>
Auto-Submit: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>