Don't try to add abstract methods to the codegen work queue to generate bodies

pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart

Index: pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
index ff5df10a44f0ce5b8e167d66305bb132aa071a33..8ced1c7ac954184bc39894a0973ee358003c6a99 100644
--- a/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
+++ b/pkg/compiler/lib/src/kernel/kernel_backend_strategy.dart
@@ -51,6 +51,8 @@ class KernelCodegenWorkItemBuilder implements WorkItemBuilder {
 
   @override
   CodegenWorkItem createWorkItem(MemberEntity entity) {
+    if (entity.isAbstract) return null;
+
     // Codegen inlines field initializers. It only needs to generate
     // code for checked setters.
     if (entity.isField && entity.isInstanceMember) {