Strengthen warnings for return types of async/async*/sync* functions.

docs/language/dartLangSpec.tex

 \documentclass{article}
 \usepackage{epsfig}
 \usepackage{color}
 \usepackage{dart}
 \usepackage{bnf}
 \usepackage{hyperref}
 \usepackage{lmodern}
 \newcommand{\code}[1]{{\sf #1}}
 \title{Dart Programming Language  Specification \\
 {\large Version 1.9}}
@@ skipping 575 matching lines @@
 \LMHash{}
 It is a compile-time error if an \ASYNC, \ASYNC* or \SYNC* modifier is attached to the body of a setter or constructor.
 
 \rationale{
 An asynchronous setter would be of little use, since setters can only be used in the context of an assignment (\ref{assignment}), and an assignment expression always evaluates to the value of the assignment's right hand side. If the setter actually did its work asynchronously, one might imagine that one would return a future that resolved to the assignment's right hand side after the setter did its work. However, this would require dynamic tests at every assignment, and so would be prohibitively expensive. 
 
 An asynchronous constructor would, by definition, never return an instance of the class it purports to construct, but instead return a future. Calling such a beast via \NEW{} would be very confusing. If you need to produce an object asynchronously, use a method.
 
 One could allow modifiers for factories. A factory for \code{Future} could be modified by \ASYNC{}, a factory for \code{Stream} could be modified by \ASYNC* and a factory for \code{Iterable} could be modified by \SYNC*. No other scenario makes sense because the object returned by the factory would be of the wrong type. This situation is very unusual so it is not worth making an exception to the general rule for constructors in order to allow it.
 }
-
+\LMHash{}
+It is a static warning if the declared return type of a function marked \ASYNC{} may not be assigned to \code{Future}. It is a static warning if the declared return type of a function marked \SYNC* may not be assigned to \code{Iterable}. It is a static warning if the declared return type of  a function marked \ASYNC* may not be assigned to \code{Stream}.  
 
 \subsection{Function Declarations}
 \LMLabel{functionDeclarations}
 
 \LMHash{}
 A {\em function declaration} is a function that is neither a member of a class nor a function literal. Function declarations include {\em library functions}, which are function declarations 
 %(including getters and setters) 
 at the top level of a library, and {\em local functions}, which are function declarations declared inside other functions. Library functions are often referred to simply as top-level functions.
 
 \LMHash{}
@@ skipping 5356 matching lines @@
 \item
 Execution of the function $m$ immediately enclosing $s$ is suspended until the method \code{moveNext()} is invoked upon the iterator used to initiate the current invocation of $m$. 
 \item
 The current call to \code{moveNext()} returns \TRUE.
 \end{itemize}
 
 \LMHash{}
 It is a compile-time error if a yield-each statement appears in a function that is not a generator function.
 
 \LMHash{}
-Let $T$ be the static type of $e$ and let $f$ be the immediately enclosing function.  It is a static type warning if $T$ may not be assigned to the declared return type of $f$. 
+Let $T$ be the static type of $e$ and let $f$ be the immediately enclosing function.  It is a static type warning if $T$ may not be assigned to the declared return type of $f$.  If $f$ is synchronous it is a static  type warning if $T$ may not be assigned to \code{Iterable}.  If $f$ is asynchronous it is a static  type warning if $T$ may not be assigned to \code{Stream}. 
 
 
 \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:}
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 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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