Make use of call on Function exempt from warnings,

BUG=
R=lrn@google.com

Review URL: https://codereview.chromium.org//1176853003.

docs/language/dartLangSpec.tex

@@ -3870,6 +3870,8 @@ \subsubsection{Ordinary Invocation}
 \item
 $T$ or a superinterface of $T$ is annotated with an annotation denoting a constant identical to the constant \code{@proxy} defined in \code{dart:core}.  Or
 \item  $T$ is \code{Type}, $e$ is a constant type literal and the class corresponding to $e$ has a static getter named $m$.
+\item $T$ is \code{Function} and $m$ is \CALL. \rationale {The type \code{Function} is treated as if it has a \code{call} method for any possible signature of \CALL. The expectation is that any concrete subclass of \code{Function} will implement \CALL. Note that a warning will be issue if this is not the case. Furthermore, any use of \CALL on a subclass of \code{Function} that fails to implement \CALL{} will also provoke a a warning, as this exemption is limited to type \code{Function}, and does not apply to its subtypes. 
+}
 \end{itemize}
 
 \LMHash{}
@@ -4158,12 +4160,14 @@ \subsubsection{General Closurization}
 \begin{itemize}
 \item $T$ or a superinterface of $T$ is annotated with an annotation denoting a constant identical to the constant \code{@proxy} defined in \cd{dart:core}. Or
 \item $T$ is \cd{Type}, $e$ is a constant type literal and the class corresponding to $e$ declares an accessible static method or getter named $m$.
+\item $T$ is \code{Function} and $m$ is \CALL.
 \end{itemize}
 
 The static type of $i$ is:
 \begin{itemize}
 \item The static type of function $T.m$, if $T$ has an accessible instance member named $m$.
 \item The static type of function $T.m$, if $T$ is \cd{Type}, $e$ is a constant type literal and the class corresponding to $e$ declares an accessible static member or constructor named $m$.
+\item \code{Function} if $T$ is \code{Function} and $m$ is \CALL.
 \item The type  \DYNAMIC{} otherwise. 
 \end{itemize}