Change formal parameter syntax to require "=" (and not ":")

A named optional parameter could previously be equipped with a default
value using ':' or '=' (as in `void f({int i : 1})` or `... i = 1 ...`).
The language has now (Dart 3.0) been updated to eliminate the form
using ':' (it has been deprecated for a long time).

This CL updates the spec_parser accordingly.

Change-Id: I28f9ec605699a9ffb2f7349a6a4b698fa709247b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/313482
Commit-Queue: Erik Ernst <eernst@google.com>
Auto-Submit: Erik Ernst <eernst@google.com>
Reviewed-by: William Hesse <whesse@google.com>
Commit-Queue: William Hesse <whesse@google.com>

tools/spec_parser/Dart.g

@@ -4,6 +4,9 @@
 
 // CHANGES:
 //
+// v0.35 Change named optional parameter syntax to require '=', that is,
+// remove the support for ':' as in `void f({int i: 1})`.
+//
 // v0.34 Add support for inline classes.
 //
 // v0.33 This commit does not change the derived language at all. It just
@@ -371,7 +374,7 @@ defaultFormalParameter
     ;
 
 defaultNamedParameter
-    :    REQUIRED? normalFormalParameter ((':' | '=') expression)?
+    :    REQUIRED? normalFormalParameter ('=' expression)?
     ;
 
 typeWithParameters