Implement override-based inference of instance fields.

pkg/front_end/testcases/inference_new/field_inference_circularity.dart

Index: pkg/front_end/testcases/inference_new/field_inference_circularity.dart
diff --git a/pkg/front_end/testcases/inference_new/field_inference_circularity.dart b/pkg/front_end/testcases/inference_new/field_inference_circularity.dart
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+++ b/pkg/front_end/testcases/inference_new/field_inference_circularity.dart
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+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/*@testedFeatures=inference*/
+library test;
+
+// A.x depends on B.x which depends on A.x, so no type is inferred.  But types
+// can be inferred for A.y and B.y.
+
+class A {
+  var /*@topType=dynamic*/ x = /*@returnType=dynamic*/ () =>
+      new B(). /*@target=B::x*/ x;
+  var /*@topType=() -> dynamic*/ y = /*@returnType=dynamic*/ () =>
+      new B(). /*@target=B::x*/ x;
+}
+
+class B extends A {
+  var /*@topType=dynamic*/ x;
+  var /*@topType=() -> dynamic*/ y;
+}
+
+main() {}