Create more synthetic kernel objects.

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

Index: pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
index cbeac83592581b5837b05554cf96d48b4bb15a06..dea40020c2d3292970e926a721eda13f5245c66e 100644
--- a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
@@ -8,6 +8,7 @@ import 'package:front_end/src/fasta/kernel/kernel_shadow_ast.dart'
     show
         KernelArguments,
         KernelComplexAssignment,
+        KernelIllegalAssignment,
         KernelIndexAssign,
         KernelPropertyAssign,
         KernelStaticAssignment,
@@ -202,7 +203,8 @@ abstract class FastaAccessor implements Accessor {
   bool get isThisPropertyAccessor => false;
 
   @override
-  KernelComplexAssignment startComplexAssignment(Expression rhs) => null;
+  KernelComplexAssignment startComplexAssignment(Expression rhs) =>
+      new KernelIllegalAssignment(rhs);
 }
 
 abstract class ErrorAccessor implements FastaAccessor {