[analyzer] Lock down the behavior of `ResultComparator` with unit tests.

In a follow-up CL, I intend to rewrite `ResultComparator` in a way
that doesn't depend on its base class, `AstComparator` (so that
`AstComparator` can be deleted). This change prepares for the rewrite
by adding unit tests for `ResultComparator`.

I've discovered some bugs in `ResultComparator` in the process:

- The failure messages reported when one of the two nodes was `null`
  were reversed. I've gone ahead and fixed this as part of this CL,
  since the fix was easy and didn't affect any tests beyond the tests
  that I've added.

- The failure message reported when a node had the wrong type
  contained an unnecessary newline. I've gone ahead and fixed this as
  part of this CL, since the fix was easy and didn't affect any tests
  beyond the tests that I've added.

- When comparing identifiers, if the actual identifier is a synthetic
  identifier, and the expected identifier is any identifier at all,
  the identifiers are considered to match. This is clearly not the
  intended behavior; the intended behavior is that the expected
  identifier must be `_s_` to match a synthetic identifier. I've
  documented this bug with a TODO, which I'll address in a follow-up
  CL.

- When comparing comments, only the references inside the comment are
  compared; the text of the comment is ignored. This seems
  inconsistent with the other behaviors of `ResultComparator`, and I'm
  pretty sure it's not necessary. I've documented this bug with a
  TODO, which I'll address in a follow-up CL.

Change-Id: Iba0169ded812d646b71966c2be22174d787c5a21
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/419424
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>