Allow `.pattern` only on const expressions.

Previously, when using the "mini-ast" pseudo-language to write unit
tests for the flow analysis and type analysis logic in the
`_fe_analyzer_shared` package, it was possible to call `.pattern` on
any arbitrary expression, creating a constant pattern containing the
expression. This didn't make sense--only constant expressions can be
used as the basis for constant patterns.

With this change, `.pattern` is only available on the various forms of
constant expressions supported by the "mini-ast" pseudo-language
(currently: integer literals, the `null` literal, and placeholder
expressions created using the `expr` function).

Change-Id: I0d87c66bdb8e636615b20bbb6207c38d78d0d2dd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/315245
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>