convert engine element setter to completion element setter

pkg/analysis_server/lib/src/computer/element.dart

Index: pkg/analysis_server/lib/src/computer/element.dart
diff --git a/pkg/analysis_server/lib/src/computer/element.dart b/pkg/analysis_server/lib/src/computer/element.dart
index f61e631b96859cb8d285c461cdeeeaf5d55092e5..2a523ed8262b528a9a086d1451fb0af28f0910f2 100644
--- a/pkg/analysis_server/lib/src/computer/element.dart
+++ b/pkg/analysis_server/lib/src/computer/element.dart
@@ -64,8 +64,12 @@ String _getParametersString(engine.Element element) {
 }
 
 String _getReturnTypeString(engine.Element element) {
-  if ((element is engine.ExecutableElement)) {
-    return element.returnType.toString();
+  if (element is engine.ExecutableElement) {
+    if (element.kind == engine.ElementKind.SETTER) {
+      return null;
+    } else {
+      return element.returnType.toString();
+    }
   } else {
     return null;
   }