Implement override-based inference of instance fields.

pkg/front_end/testcases/inference/infer_field_override_multiple.dart

+// 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;
+
+abstract class A {
+  int get x;
+}
+
+abstract class B {
+  int get x;
+}
+
+abstract class C {
+  num get x;
+}
+
+abstract class D {
+  double get x;
+}
+
+// Superclasses have a consistent type for `x` so inferrence succeeds.
+class E extends A implements B {
+  var /*@topType=int*/ x;
+}
+
+// Superclasses don't have a consistent type for `x` so inference fails, even if
+// the types are related.
+class F extends A implements C {
+  var /*@topType=dynamic*/ x;
+}
+
+class G extends A implements D {
+  var /*@topType=dynamic*/ x;
+}
+
+class H extends C implements D {
+  var /*@topType=dynamic*/ x;
+}
+
+main() {}