Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_field_override_of_override.dart

Index: pkg/front_end/testcases/inference/infer_field_override_of_override.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_of_override.dart
similarity index 54%
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_of_override.dart
index c8bf6f91deec52a3838f850b10dc3a222cb71835..c1438e7313be5a556e7a70d0c36359d4a501497c 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_of_override.dart
@@ -5,15 +5,16 @@
 /*@testedFeatures=inference*/
 library test;
 
-class A {
-  var /*@topType=dynamic*/ x;
+abstract class A {
+  num get x;
 }
 
-class B implements A {
-  var /*@topType=dynamic*/ x = 2;
+abstract class B extends A {
+  int get x;
 }
 
-foo() {
-  String y = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
-  int z = /*info:DYNAMIC_CAST*/ new B(). /*@target=B::x*/ x;
+class C extends B {
+  var /*@topType=int*/ x;
 }
+
+main() {}