Revise spec language on warnings. (#3584)

Incorporates feedback from:

#3570.

specification/dartLangSpec.tex

@@ -1268,8 +1268,8 @@ \section{Errors and Warnings}
 but which are unlikely to be intended,
 and likely to cause bugs or inconveniences.
 A Dart compiler is free to report some, all,
-or none of the specified static warnings before executing the associated code.
-A Dart compiler may also report additional warnings
+or none of the specified static warnings before the associated code is executed.
+A Dart compiler may also choose to report additional warnings
 not defined by this specification.
 
 \LMHash{}%