In order to make an implementation of initializing formal access in
`dart2js` available as specified in issue #26655 now, this CL changes
the scope management such that initializing formals are not in scope in
the constructor body, only in the initializer list.
This is done by introducing a new notion of scopes implemented
by `ExtensionScope`: Such a scope will extend an existing
`NestedScope` rather than adding a new nested scope to it.
Review URL: https://codereview.chromium.org/2059883002 .