Implement override-based inference of instance fields.

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

+library test;
+import self as self;
+import "dart:core" as core;
+
+class A extends core::Object implements self::B {
+  field core::int x = null;
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+class B extends core::Object {
+  field core::int x = 0;
+  constructor •() → void
+    : super core::Object::•()
+    ;
+}
+static method main() → dynamic {}