Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_field_override_with_substitution.dart

Index: pkg/front_end/testcases/inference/infer_field_override_with_substitution.dart
diff --git a/pkg/front_end/testcases/inference/infer_from_rhs_only_if_it_wont_conflict_with_overridden_fields.dart b/pkg/front_end/testcases/inference/infer_field_override_with_substitution.dart
similarity index 53%
copy from pkg/front_end/testcases/inference/infer_from_rhs_only_if_it_wont_conflict_with_overridden_fields.dart
copy to pkg/front_end/testcases/inference/infer_field_override_with_substitution.dart
index c8bf6f91deec52a3838f850b10dc3a222cb71835..966ead89c922ecef6720c9c58073b7ecbface9d3 100644
--- a/pkg/front_end/testcases/inference/infer_from_rhs_only_if_it_wont_conflict_with_overridden_fields.dart
+++ b/pkg/front_end/testcases/inference/infer_field_override_with_substitution.dart
@@ -5,15 +5,16 @@
 /*@testedFeatures=inference*/
 library test;
 
-class A {
-  var /*@topType=dynamic*/ x;
+abstract class A<T> {
+  List<T> get x;
+  void set y(List<T> value);
+  List<T> z;
 }
 
-class B implements A {
-  var /*@topType=dynamic*/ x = 2;
+class B extends A<int> {
+  var /*@topType=List<int>*/ x;
+  var /*@topType=List<int>*/ y;
+  var /*@topType=List<int>*/ z;
 }
 
-foo() {
-  String y = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
-  int z = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
-}
+main() {}