Fix firstNonCommentToken in anticipation of analyzer fix. (#1503)

This commit fixes a subtle bug in the formatter which is currently
covered up by an analyzer bug. The formatter bug is this:
`AstNodeExtensions.firstNonCommentToken` assumes that if an AST node
doesn't implement `AnnotatedNode`, then it can't possibly begin with a
documentation comment, so it is safe to simply defer to
`beginToken`. But that isn't true `DefaultFormalParameter` doesn't
implement `AnnotatedNode`, but its first child is a
`NormalFormalParameter`, which *can* begin with a documentation
comment.

Currently, analyzer bug dart-lang/sdk#56313
prevents `NormalFormalParameter.beginToken` from properly accounting
for the presence of the documentation comment, so as a result,
`AstNodeExtensions.firstNonCommentToken` works correctly on
`DefaultFormalParameter`. In order to avoid breaking the formatter
when I fix the analyzer bug, I need to first fix the formatter.

The formatter fix is for `AstNodeExtensions.firstNonCommentToken` to
recognize that `DefaultFormalParameter` is unusual, and to compute its
first non-comment token directly (by looking at the first non-cmoment
token of its first child, `parameter`). The formatter fix works
regardless of whether the analyzer bug is fixed, so it is safe to land
the formatter fix first.

Author: stereotype441

lib/src/ast_extensions.dart

@@ -18,6 +18,9 @@ extension AstNodeExtensions on AstNode {
     return switch (this) {
       AnnotatedNode(metadata: [var annotation, ...]) => annotation.beginToken,
       AnnotatedNode(firstTokenAfterCommentAndMetadata: var token) => token,
+      // DefaultFormalParameter is not an AnnotatedNode, but its first child
+      // (parameter) *is* an AnnotatedNode, so we can't just use beginToken.
+      DefaultFormalParameter(:var parameter) => parameter.firstNonCommentToken,
       _ => beginToken
     };
   }