Revise static typecheck rules for for-in.

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 1.10}}
@@ skipping 5665 matching lines @@
 \LMHash{}
 A for statement of the form \code{ \FOR{} ($finalConstVarOrType?$ id \IN{} $e$) $s$} is equivalent to the following code: 
 
 \begin{dartCode}
 var n0 = $e$.iterator;
 \WHILE{} (n0.moveNext()) \{
    $finalConstVarOrType?$ id = n0.current;
    $s$ 
 \}
 \end{dartCode}
-where \code{n0} is an identifier that does not occur anywhere in the program.
+where \code{n0} is an identifier that does not occur anywhere in the program, except that for purposes of static typechecking, it is checked under the assumption that $n0$ is declared to be of type $T$, where $T$ is the static type of $e.iterator$.
 
-\commentary{
-Note that in fact, using a  \CONST{} variable would give rise to a compile time error since \cd{n0.current} is not a constant expression.  
-}
-
-It is a static warning if the static type of $e$ does not have a member \cd{iterator}. It is a static warning if the static type of \cd{$e$.iterator} does not have a method \cd{moveNext}. It is a static warning if static return type of  \cd{$e$.iterator.moveNext()} is not assignable to the static type of \cd{id}.
-
-
-\commentary {
-One might have required that \cd{n0} had type \cd{Iterable$<T>$} where $T$ was the static type of \cd{id}. However, that would cause failure in checked mode if $e$ was merely emulating the  \cd{Iterable} interface. 
-}
  
  
 \subsubsection{Asynchronous For-in}
 \LMLabel{asynchronousFor-in}
 
 \LMHash{}
 A for-in statement may be asynchronous. The asynchronous form is designed to iterate over streams. An asynchronous for loop is distinguished by the keyword \AWAIT{} immediately preceding the keyword \FOR.
 
 \LMHash{}
 Execution of a for-in statement of the form  \code{\AWAIT{} \FOR{} (finalConstVarOrType? id \IN{} $e$) $s$} proceeds as follows:
@@ skipping 2137 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.
 
 ----------------------------------------------------------------------