Issue 30179. Infer return type of static setters to 'void'.

pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.kernel_procedure_builder;
 
 import 'package:kernel/ast.dart'
     show
         Arguments,
         AsyncMarker,
@@ skipping 233 matching lines @@
   void set asyncModifier(AsyncMarker newModifier) {
     actualAsyncModifier = newModifier;
     if (function != null) {
       // No parent, it's an enum.
       function.asyncMarker = actualAsyncModifier;
       function.dartAsyncMarker = actualAsyncModifier;
     }
   }
 
   bool get isEligibleForTopLevelInference {
-    if (!isInstanceMember) return false;
-    if (returnType == null) return true;
-    if (formals != null) {
-      for (var formal in formals) {
-        if (formal.type == null) return true;
+    if (isInstanceMember) {
+      if (returnType == null) return true;
+      if (formals != null) {
+        for (var formal in formals) {
+          if (formal.type == null) return true;
+        }
       }
+    } else {
+      if (isSetter && returnType == null) return true;
     }
     return false;
   }
 
   Procedure build(SourceLibraryBuilder library) {
     // TODO(ahe): I think we may call this twice on parts. Investigate.
     if (procedure.name == null) {
       procedure.function = buildFunction(library);
       procedure.function.parent = procedure;
       procedure.function.fileOffset = charOpenParenOffset;
@@ skipping 139 matching lines @@
         }
       }
       initializers.add(initializer..parent = constructor);
       initializers.add(superInitializer);
       return;
     }
     initializers.add(initializer);
     initializer.parent = constructor;
   }
 }