Added null-shorting rule to treat selector ! like other selectors (#1162)

Discussions about this decision can be found in #1163.

Author: eernstg

accepted/future-releases/nnbd/feature-specification.md

@@ -6,6 +6,9 @@ Status: Draft
 
 ## CHANGELOG
 
+2020.10.14
+  - Include selector `!` among the null-shorting constructs.
+
 2020.10.12
   - Clarify that operators not mentioned explicitly in the rules
     do not participate in the null-shorting transformation.
@@ -1336,6 +1339,8 @@ continuation.
   - `SHORT[EXP(e1), fn[x] => x[EXP(e2)]]`
 - If `e1` translates to `F` then `e1[e2]` translates to:
   - `PASSTHRU[F, fn[x] => x[EXP(e2)]]`
+- If `e` translates to `F` then `e!` translates to:
+  - `PASSTHRU[F, fn[x] => x!]`
 - The assignment `e1?.f = e2` translates to:
   - `SHORT[EXP(e1), fn[x] => x.f = EXP(e2)]`
 - The other assignment operators are handled equivalently.