[cfe] Provide a helper for computing interface member signature type

Change-Id: Iefad05b1a5e9b6f93f821ad29635af31e6c8926e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/381642
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Ömer Ağacan <omersa@google.com>

pkg/kernel/lib/ast.dart

@@ -2979,7 +2979,10 @@ class Procedure extends Member implements GenericFunction {
   /// this, it is stored explicitly as the [signatureType] on the procedure.
   ///
   /// When [signatureType] is null, you can compute the function type with
-  /// `function.computeFunctionType(Nullability.nonNullable)`.
+  /// `function.computeFunctionType(Nullability.nonNullable)`. Alternatively,
+  /// you can use [computeSignatureOrFunctionType] that computes the interface
+  /// member signature type accounting for the possibility of [signatureType]
+  /// being null.
   FunctionType? signatureType;
 
   Procedure(Name name, ProcedureKind kind, FunctionNode function,
@@ -3234,6 +3237,15 @@ class Procedure extends Member implements GenericFunction {
         : (flags & ~FlagHasWeakTearoffReferencePragma);
   }
 
+  /// Computes the interface member signature type of the procedure.
+  ///
+  /// In case [signatureType] is set, returns [signatureType]. Otherwise,
+  /// computes the function type of the function node.
+  FunctionType computeSignatureOrFunctionType() {
+    return signatureType ??
+        function.computeFunctionType(Nullability.nonNullable);
+  }
+
   @override
   R accept<R>(MemberVisitor<R> v) => v.visitProcedure(this);