In strong mode, properly infer the return types of setters as `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:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show KernelProcedure;
 
 import 'package:front_end/src/fasta/source/source_library_builder.dart'
@@ skipping 229 matching lines @@
     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) {
-      // For now, we skip inferred return types of setters.
-      // TODO(paulberry): fix this.
-      if (!isSetter) return true;
-    }
+    if (returnType == null) return true;
     if (formals != null) {
       for (var formal in formals) {
         if (formal.type == null) return true;
       }
     }
     return false;
   }
 
   Procedure build(SourceLibraryBuilder library) {
     // TODO(ahe): I think we may call this twice on parts. Investigate.
@@ skipping 145 matching lines @@
         }
       }
       initializers.add(initializer..parent = constructor);
       initializers.add(superInitializer);
       return;
     }
     initializers.add(initializer);
     initializer.parent = constructor;
   }
 }