Revised all spec occurrences of "dynamic error"/"dynamic type error"

.. and checked that they can occur also in Dart 2; added a comment
about why this is so, on each of them. Also, got rid of the term
"run-time error": the majority of references to errors at run time
were "dynamic error", and now that is the term used everywhere.

Cf. #34521.

Change-Id: I7579c84a8d52199524770fb91c64804173ed533d
Reviewed-on: https://dart-review.googlesource.com/c/90243
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

docs/language/dart.sty

@@ -133,6 +133,11 @@
 \newcommand{\IndexCustom}[2]{%
   \leavevmode\marginpar{\ensuremath{\diamond}}\emph{#1}\index{#2}}
 
+% Used when one concept should have >1 entry in the index. Does not add
+% the diamond in the margin and shows no text where the command occurs.
+% Intended to be used immediately after another \Index... command.
+\newcommand{\IndexExtraEntry}[1]{\index{#1}}
+
 % Used for a defining occurrence of a phrase, adding it to the index.
 \newcommand{\Index}[1]{\IndexCustom{#1}{#1}}