Add type inference for complex assignments whose LHS is an index expression.

pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
index 9d575b161bd91831da70b628b53a00e80125bb80..7123076acfc0d99e0003b4f086255be60fefc81c 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
@@ -156,6 +156,12 @@ class TypeInferenceListener
   void ifStatementExit(IfStatement statement) =>
       debugStatementExit('ifStatement', statement);
 
+  bool indexAssignEnter(Expression expression, DartType typeContext) =>
+      debugExpressionEnter("indexAssign", expression, typeContext);
+
+  void indexAssignExit(Expression expression, DartType inferredType) =>
+      debugExpressionExit("indexAssign", expression, inferredType);
+
   bool intLiteralEnter(IntLiteral expression, DartType typeContext) =>
       debugExpressionEnter("intLiteral", expression, typeContext);