Allow doc comments to use formal parameter initializer scope (#3494)

Allow doc comments to use the formal parameter initializer scope. In particular, a parameter of the form `this.name` is then in scope, as well as everything which was in scope before this change.

Author: parlough

specification/dartLangSpec.tex

@@ -23076,7 +23076,14 @@ \subsubsection{Comments}
 
 \LMHash{}%
 The current scope for a documentation comment immediately preceding
-the declaration of a function $f$ is the formal parameter scope of $f$.
+the declaration of a non-redirecting generative constructor $k$
+with initializing formals is the formal parameter initializer scope of $k$
+(\ref{generativeConstructors}).
+
+\LMHash{}%
+Otherwise, the current scope for a documentation comment immediately preceding
+the declaration of a function $f$ is the formal parameter scope of $f$
+(\ref{formalParameters}).
 
 
 \subsection{Operator Precedence}