Adjust Dart.g to allow prefix on Function as a type

This CL changes the specification grammar Dart.g such that it allows
using `p.Function` to denote the built-in type `Function`. This is
necessary in the case where 'dart:core' has been imported with an
import prefix `p` (and it hasn't been imported without a prefix at the
same time). The corresponding spec update is in
dart-lang/language#3492.

Change-Id: Iaaba1c3bc8f5d3a8700c8e62ec3a1af92334840b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/339561
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Erik Ernst <[email protected]>

Author: eernstg

tools/spec_parser/Dart.g

@@ -1441,7 +1441,7 @@ typeNotFunction
 
 typeNotVoidNotFunction
     :    typeName typeArguments?
-    |    FUNCTION
+    |    (typeIdentifier '.')? FUNCTION
     ;
 
 typeName