Fix type inference of getters that "override" setters and vice versa.

pkg/front_end/testcases/inference/infer_setter_cross_to_getter.dart.strong.expect

Index: pkg/front_end/testcases/inference/infer_setter_cross_to_getter.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/infer_setter_cross_to_getter.dart.strong.expect b/pkg/front_end/testcases/inference/infer_setter_cross_to_getter.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..ce71a72cc095438adbfbb56deb1e4cd86bd469b9
--- /dev/null
+++ b/pkg/front_end/testcases/inference/infer_setter_cross_to_getter.dart.strong.expect
@@ -0,0 +1,17 @@
+library test;
+import self as self;
+import "dart:core" as core;
+
+abstract class A extends core::Object {
+  constructor •() → void
+    : super core::Object::•()
+    ;
+  abstract get x() → core::double;
+}
+abstract class B extends self::A {
+  constructor •() → void
+    : super self::A::•()
+    ;
+  abstract set x(core::double value) → void;
+}
+static method main() → dynamic {}