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

pkg/front_end/testcases/inference/infer_assign_to_property.dart

Index: pkg/front_end/testcases/inference/infer_assign_to_property.dart
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_property.dart b/pkg/front_end/testcases/inference/infer_assign_to_property.dart
index 16e394f188c622072d9e0034c908fdbfb30adf7d..a3777f50181d14576f8cc97c9488c7805fee4378 100644
--- a/pkg/front_end/testcases/inference/infer_assign_to_property.dart
+++ b/pkg/front_end/testcases/inference/infer_assign_to_property.dart
@@ -17,10 +17,10 @@ var /*@topType=dynamic*/ v_minus =
     (/*error:TOP_LEVEL_UNSUPPORTED*/ new A(). /*@target=A::f*/ f -= 1);
 var /*@topType=dynamic*/ v_multiply =
     (/*error:TOP_LEVEL_UNSUPPORTED*/ new A(). /*@target=A::f*/ f *= 1);
-var /*@topType=dynamic*/ v_prefix_pp = (/*@target=num::+*/ ++new A()
-    . /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
-var /*@topType=dynamic*/ v_prefix_mm = (/*@target=num::-*/ --new A()
-    . /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
+var /*@topType=dynamic*/ v_prefix_pp =
+    (++new A(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
+var /*@topType=dynamic*/ v_prefix_mm =
+    (--new A(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f);
 var /*@topType=dynamic*/ v_postfix_pp =
     (new A(). /*error:TOP_LEVEL_INSTANCE_GETTER*/ /*@target=A::f*/ f++);
 var /*@topType=dynamic*/ v_postfix_mm =