Add documentation to the mixin superclass requirement description.

docs/language/dartLangSpec.tex

Index: docs/language/dartLangSpec.tex
diff --git a/docs/language/dartLangSpec.tex b/docs/language/dartLangSpec.tex
index 4b2c5148241d41febb06341bc0767c2e7050a4b4..e826260a5cd9f486b591310eaeac74e59a0a9ff6 100644
--- a/docs/language/dartLangSpec.tex
+++ b/docs/language/dartLangSpec.tex
@@ -2083,10 +2083,9 @@ If, for example, $M$ declares an instance member $im$ whose type is at odds with
 }
 
 \LMHash{}
-The effect of a class definition of the form \code{\CLASS{} $C$ = $M$; } or the form
- \code{\CLASS{} $C<T_1, \ldots, T_n>$ = $M$; } in library $L$  is to introduce the name $C$ into the scope of $L$, bound to the class (\ref{classes}) defined by the mixin application $M$. The name of the class is also set to $C$. Iff the  class is prefixed by the built-in identifier \ABSTRACT{}, the class being defined is an abstract class.
+The effect of a class definition of the form \code{\CLASS{} $C$ = $M$; } or the form \code{\CLASS{} $C<T_1, \ldots, T_n>$ = $M$; } in library $L$  is to introduce the name $C$ into the scope of $L$, bound to the class (\ref{classes}) defined by the mixin application $M$. The name of the class is also set to $C$. Iff the  class is prefixed by the built-in identifier \ABSTRACT{}, the class being defined is an abstract class.

[eernst, 2016/09/28 12:20:31]

OK, let's keep it like this: It looks like this is the removal of an unmotivated
newline, but that's not enough of a reason to split the paragraph into smaller
pieces (e.g., newline at each end-of-sentence), as earlier discussed.

[Lasse Reichstein Nielsen, 2016/09/29 06:54:50]

Acknowledged.

 
- Let $M_A$ be a mixin derived from a class $M$ with direct superclass $S_{static}$.
+Let $M_A$ be a mixin derived from a class $M$ with direct superclass $S_{static}$, e.g., as defined by the class declaration \code{class M extends S$_{static}$ \{ \ldots \}}.
 
 Let $A$ be an application of $M_A$. It is a static warning if the superclass of $A$ is not a subtype of $S_{static}$.
 
@@ -2872,7 +2871,7 @@ A {\em symbol literal} denotes the name of a declaration in a Dart program.
 A symbol literal \code{\#id} where \code{id} does not begin with an underscore ('\code{\_}')  is equivalent to the expression \code{\CONST{} Symbol('id')}.
 
 \LMHash{}
-A symbol literal \code{\#\_id} evaluates to the object that would be returned by the call \code{MirrorSystem.getSymbol('_id', libraryMirror)} where \code{libraryMirror} is an instance of the class \code{LibraryMirror} defined in the library \code{dart:mirrors}, reflecting the current library.
+A symbol literal \code{\#\_id} evaluates to the object that would be returned by the call \code{MirrorSystem.getSymbol("\_id", libraryMirror)} where \code{libraryMirror} is an instance of the class \code{LibraryMirror} defined in the library \code{dart:mirrors}, reflecting the current library.

[eernst, 2016/09/28 12:20:31]

Thinking .. aha, this is a fix to the fix (it used to say `getPrivateSymbol`):
OK

[Lasse Reichstein Nielsen, 2016/09/29 06:54:50]

Acknowledged.

 
 \rationale{
 One may well ask what is the motivation for introducing literal symbols? In some languages, symbols are canonicalized whereas strings are not. However literal strings are already canonicalized in Dart.  Symbols are slightly easier to type compared to strings and their use can become strangely addictive, but this is not nearly sufficient justification for adding a literal form to the language. The primary motivation is related to the use of reflection and a web specific practice known as minification.