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. dart-lang/sdk#34521.

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

subtyping.md

@@ -0,0 +1,9 @@
+# Dart 2.0 Static and Runtime Subtyping
+
+leafp@google.com
+
+**Status**: This document has been integrated into the language specification.
+Also, an updated version taking non-null types into account exists
+[here](https://github.com/dart-lang/language/blob/master/resources/type-system/subtyping.md).
+
+**Contents of this document**: Deleted.