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

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 78420049dd2423fb4dca874cf50e60568dbb7fb0..29b27b51adecf4508008aff5e7d529bf5fa0e5f6 100644
--- a/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/fasta_accessors.dart
@@ -817,6 +817,10 @@ class SuperIndexAccessor extends kernel.SuperIndexAccessor with FastaAccessor {
   }
 
   toString() => "SuperIndexAccessor()";
+
+  @override
+  KernelComplexAssignment startComplexAssignment(Expression rhs) =>
+      new KernelIndexAssign(null, index, rhs, isSuper: true);
 }
 
 class ThisPropertyAccessor extends kernel.ThisPropertyAccessor