Clarify that dart system libs have lower priority regardless how imported.

docs/language/dartLangSpec.tex

Index: docs/language/dartLangSpec.tex
===================================================================
--- docs/language/dartLangSpec.tex	(revision 34260)
+++ docs/language/dartLangSpec.tex	(working copy)
@@ -5031,9 +5031,9 @@
 
 \rationale{The greatly increases the chance that a member can be added to a library without breaking its importers.}
 
-If a name $N$ is referenced by a library $L$ and $N$ would be introduced into the top level scope of $L$ by an import from a library whose URI begins with \code{dart:} and an import from a library whose URI does not begin with \code{dart:}:
+A {\em system library} is a library whose URI begins with \code{dart:}. Any other library is {\em non-system library}. If a name $N$ is referenced by a library $L$ and $N$ would be introduced into the top level scope of $L$ by an import of a  system library and an import of a non-system library:

[kasperl, 2014/03/24 07:48:14]

I don't think this is enough. We need to talk about the element that the name N
refers to. If the name would be introduced into the top-level scope of L by the
import of a library I1 and another library I2 and that the definition of N
pulled in via the import of I1 is (indirectly through potentially long re-export
chains) from a system library then we want to hide the name N on the import of
I1.

 \begin{itemize}
-\item The import from \code{dart:} is implicitly extended by a \code{\HIDE{} $N$} clause.
+\item The import of the system library is implicitly extended by a \code{\HIDE{} $N$} clause.
 \item A static warning is issued.
 \end{itemize}
 
@@ -5133,9 +5133,9 @@
 For each
 entry mapping key $k$ to declaration $d$ in $NS_n$ an entry mapping $k$ to $d$ is added to the exported namespace of $L$ unless a  top-level declaration with the name $k$ exists in $L$.  
 
-If a name $N$ is referenced by a library $L$ and $N$ would be introduced into the exported namespace of $L$ by an export from a library whose URI begins with \code{dart:} and an export from a library whose URI does not begin with \code{dart:}:
+If a name $N$ is referenced by a library $L$ and $N$ would be introduced into the exported namespace of $L$ by an export of a system library and an export of a non-system library:
 \begin{itemize}
-\item The export from \code{dart:} is implicitly extended by a \code{\HIDE{} $N$} clause.
+\item The export of the system library is implicitly extended by a \code{\HIDE{} $N$} clause.
 \item A static warning is issued.
 \end{itemize}