Enable top level inference of instance property gets/sets.

pkg/front_end/testcases/inference_new/infer_instance_field_ref.dart.strong.expect

+library test;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object {
+  field self::B b = null;
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class B extends core::Object {
+  field self::C c = null;
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class C extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class D extends self::C {
+  constructor •() → void
+    : super self::C::•()
+    ;
+}
+static field self::A a = new self::A::•();
+static field self::C x = self::a.{self::A::b}.{self::B::c};
+static field self::C y = let final dynamic #t1 = self::a.{self::A::b} in let final dynamic #t2 = #t1.{self::B::c} in #t2.{core::Object::==}(null) ? #t1.{self::B::c} = new self::D::•() : #t2;
+static method main() → dynamic {}