In strong mode, properly infer the return types of setters as `void`

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
index 6e35f2bb3faab50a1be03b25d1b652ed71ef01cb..05ab9f60e3694907eb77093ee2aec8807f0b294f 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart
@@ -1518,6 +1518,17 @@ class KernelProcedure extends Procedure implements KernelMember {
   static bool hasImplicitReturnType(KernelProcedure procedure) {
     return procedure._hasImplicitReturnType;
   }
+
+  static void inferSetterReturnType(
+      KernelProcedure procedure, TypeInferenceEngineImpl engine, String uri) {
+    assert(procedure.isSetter);
+    if (procedure._hasImplicitReturnType) {
+      var inferredType = const VoidType();
+      engine.instrumentation?.record(Uri.parse(uri), procedure.fileOffset,
+          'topType', new InstrumentationValueForType(inferredType));
+      procedure.function?.returnType = inferredType;
+    }
+  }
 }
 
 /// Concrete shadow object representing an assignment to a property.