Added metadata to new function type syntax parameter specifications

In #30732 the concern was raised
that the new Function type syntax does not support metadata on
parameter specifications (i.e., on normalParameterTypes and on
namedParameterTypes).

The implication of adding support for metadata in these locations is
that `@required` can be used on function types, which is the motivation
for submitting 30732.

We have always had support on parameter declarations in function typed
parameter declarations (`void foo(@A() int f(@A() String s))`), so in
this sense there is no new semantics to worry about (Lasse: "it doesn't
mean anything anyway!").

This CL modifies the generic-function-type-alias.md informal spec to
include this kind of metadata support.

Change-Id: I4520d330458242b31c991f62c03ca2f34f9c5e54
Reviewed-on: https://dart-review.googlesource.com/5762
Commit-Queue: Erik Ernst <[email protected]>
Reviewed-by: Bob Nystrom <[email protected]>
Reviewed-by: Lasse R.H. Nielsen <[email protected]>

docs/language/informal/generic-function-type-alias.md

@@ -132,13 +132,15 @@ parameterTypeList: // NEW
 normalParameterTypes: // NEW
   normalParameterType (',' normalParameterType)*
 normalParameterType: // NEW
-  type | typedIdentifier
+  metadata (type | typedIdentifier)
 optionalParameterTypes: // NEW
   optionalPositionalParameterTypes | namedParameterTypes
 optionalPositionalParameterTypes: // NEW
   '[' normalParameterTypes ','? ']'
 namedParameterTypes: // NEW
-  '{' typedIdentifier (',' typedIdentifier)* ','? '}'
+  '{' namedParameterType (',' namedParameterType)* ','? '}'
+namedParameterType: // NEW
+  metadata typedIdentifier
 typedIdentifier: // NEW
   type identifier
 type: // CHANGED