Implement override-based inference of instance fields.

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

Index: pkg/front_end/testcases/inference/infer_field_override_of_override.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/infer_field_override_of_override.dart.strong.expect b/pkg/front_end/testcases/inference/infer_field_override_of_override.dart.strong.expect
new file mode 100644
index 0000000000000000000000000000000000000000..1362eada0985339bb8df6440e01321b1d5c8ee83
--- /dev/null
+++ b/pkg/front_end/testcases/inference/infer_field_override_of_override.dart.strong.expect
@@ -0,0 +1,23 @@
+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::num;
+}
+abstract class B extends self::A {
+  constructor •() → void
+    : super self::A::•()
+    ;
+  abstract get x() → core::int;
+}
+class C extends self::B {
+  field core::int x = null;
+  constructor •() → void
+    : super self::B::•()
+    ;
+}
+static method main() → dynamic {}