Add message to assert in specification.

docs/language/dartLangSpec.tex

 \documentclass{article}
 \usepackage{epsfig}
 \usepackage{color}
 \usepackage{dart}
 \usepackage{bnf}
 \usepackage{hyperref}
 \usepackage{lmodern}
 \usepackage[T1]{fontenc}
 \newcommand{\code}[1]{{\sf #1}}
 \title{Dart Programming Language  Specification  \\
@@ skipping 6575 matching lines @@
 
 
 \subsection{ Assert}
 \LMLabel{assert}
 
 \LMHash{}
 An {\em assert statement} is used to disrupt normal execution if a given boolean condition does not hold.
 
 \begin{grammar}
 {\bf assertStatement:}
-   assert `(' expression `)' `{\escapegrammar ;}'
+   assert `(' expression ( `,` expression )? `)' `{\escapegrammar ;}'
       .
 \end{grammar}
 
 \LMHash{}
-The assert statement has no effect in production mode. In checked mode, execution of an assert statement \code{\ASSERT{}($e$);} proceeds as follows:
+An assert statement on the form \code{\ASSERT($e$);)} is equivalent to a statment on the form \code{\ASSERT($e$, null);}.
 
 \LMHash{}
-The expression $e$ is evaluated to an object $o$. If the class of $o$ is a subtype of \code{Function} then let $r$ be the result of invoking $o$ with no arguments. Otherwise, let $r$ be $o$.
-It is a dynamic type error if $o$ is not of type \code{bool} or of type \code{Function}, or if $r$ is not of type \code{bool}.  If $r$ is \FALSE{}, we say that the assertion failed. If $r$ is \TRUE{}, we say that the assertion succeeded. If the assertion succeeded, execution of the assert statement is complete. If the assertion failed, the execution throws (\ref{completion}) an \code{AssertionError} with a stack trace corresponding to the current execution state at the \ASSERT{} statement.
+The assert statement has no effect in production mode. In checked mode, execution of an assert statement \code{\ASSERT{}($c$, $e$);} proceeds as follows:
 
-%\Q{Might be cleaner to define it as \code{if (!$e$) \{\THROW{} \NEW{} AssertionError();\}} (in checked mode only).
-%What about an error message as part of the assert?}
+\LMHash{}
+The expression $c$ is evaluated to an object $o$. If the class of $o$ is a subtype of \code{Function} then let $r$ be the result of invoking $o$ with no arguments. Otherwise, let $r$ be $o$.
+It is a dynamic type error if $o$ is not of type \code{bool} or of type \code{Function}, or if $r$ is not of type \code{bool}.
+If $r$ is \FALSE{}, we say that the assertion failed.
+If $r$ is \TRUE{}, we say that the assertion succeeded.
+If the assertion succeeded, execution of the assert statement completes normally (\ref{completion}).
+If the assertion failed, $e$ is evaluated to an object $m$.
+Then the execution of the assert statement throws (\ref{completion}) an \code{AssertionError} containing $m$ and with a stack trace corresponding to the current execution state at the \ASSERT{} statement.
 
 \LMHash{}
  It is a static type warning if the type of $e$ may not be assigned to either  \code{bool} or $() \rightarrow$ \code{bool}.
 
 \rationale{Why is this a statement, not a built in function call? Because it is handled magically so it has no effect and no overhead in production mode. Also, in the absence of final methods. one could not prevent it being overridden (though there is no real harm in that).  It cannot be viewed as a function call that is being optimized away because the argument might have side effects.
 }
 
 %If a lexically visible declaration named \code{assert} is in scope, an assert statement
 %\code{\ASSERT{} (e); }
 %is interpreted as an expression statement \code{(assert(e));} .
@@ skipping 1387 matching lines @@
 
 The invariant that each normative paragraph is associated with a line
 containing the text \LMHash{} should be maintained.  Extra occurrences
 of \LMHash{} can be added if needed, e.g., in order to make
 individual \item{}s in itemized lists addressable.  Each \LM.. command
 must occur on a separate line.  \LMHash{} must occur immediately
 before the associated paragraph, and \LMLabel must occur immediately
 after the associated \section{}, \subsection{} etc.
 
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