Add type inference for assignments to `this[...]`

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 29b27b51adecf4508008aff5e7d529bf5fa0e5f6..8b72ce58520473e80e5186266c3fee2625d4e0ff 100644
--- a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
@@ -799,6 +799,10 @@ class ThisIndexAccessor extends kernel.ThisIndexAccessor with FastaAccessor {
   }
 
   toString() => "ThisIndexAccessor()";
+
+  @override
+  KernelComplexAssignment startComplexAssignment(Expression rhs) =>
+      new KernelIndexAssign(null, index, rhs);
 }
 
 class SuperIndexAccessor extends kernel.SuperIndexAccessor with FastaAccessor {