Make ?? a compile-time constant operator.

docs/language/dartLangSpec.tex

Index: docs/language/dartLangSpec.tex
diff --git a/docs/language/dartLangSpec.tex b/docs/language/dartLangSpec.tex
index 3850f200dfcfc00dea26f2a7b80fdf71a2c7f3a8..fb3a9e47bbf7d58ccfde77707b0a53d74f5efc89 100644
--- a/docs/language/dartLangSpec.tex
+++ b/docs/language/dartLangSpec.tex
@@ -2435,6 +2435,7 @@ A constant expression is one of the following:
 \item An expression of the form \code{$e_1 + e_2$} where $e_1$ and $e_2$ are constant expressions that evaluate to a numeric or string value or to \NULL{}.
 \item An expression of one of the forms \code{$-e$}, \code{$e_1$ - $e_2$}, \code{$e_1$ * $e_2$}, \code{$e_1$ / $e_2$,} \code{$e_1$ \~{}/ $e_2$},  \code{$e_1  >  e_2$}, \code{$e_1  <  e_2$}, \code{$e_1$ $>$= $e_2$}, \code{$e_1$ $<$= $e_2$} or \code{$e_1$ \% $e_2$},  where $e$, $e_1$ and $e_2$ are constant expressions that evaluate to a numeric value  or to \NULL{}.
 \item An expression of the form \code{$e_1$?$e_2$:$e3$} where $e_1$, $e_2$ and $e_3$ are constant expressions and $e_1$ evaluates to a boolean value.
+\item An expression of the form \code{$e_1 ?? e_2$} where $e_1$ and $e_2$ are constant expressions.
 \item An expression of the form \code{$e$.length} where $e$ is a constant expression that evaluates to a string value.
 \end{itemize}