[cfe] Move FieldDeclaration et al. to their own libraries

Change-Id: I7d086497e4057bd73988772ccdc09e731375a01e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/424942
Reviewed-by: Chloe Stefantsova <[email protected]>
Commit-Queue: Johnni Winther <[email protected]>

Author: johnniwinther

pkg/front_end/lib/src/fragment/enum_element.dart

@@ -578,19 +578,19 @@ class EnumElementDeclaration
 
   @override
   // Coverage-ignore(suite): Not run.
-  DartType get _fieldTypeInternal => _type;
+  DartType get fieldTypeInternal => _type;
 
   @override
   bool get isConst => true;
 
   @override
   // Coverage-ignore(suite): Not run.
-  void _setCovariantByClassInternal() {
+  void setCovariantByClassInternal() {
     _field!.isCovariantByClass = true;
   }
 
   @override
-  void set _fieldTypeInternal(DartType value) {
+  void set fieldTypeInternal(DartType value) {
     _type = value;
     _field?.type = value;
   }