Implement type inference of getters/setters based on inheritance.

pkg/front_end/testcases/inference/conflicts_can_happen.dart

Index: pkg/front_end/testcases/inference/conflicts_can_happen.dart
diff --git a/pkg/front_end/testcases/inference/conflicts_can_happen.dart b/pkg/front_end/testcases/inference/conflicts_can_happen.dart
index e4ef920212725a559478a81e2c1d337cadc52ff9..8e7290872910c05276c03f628f231593decaa03e 100644
--- a/pkg/front_end/testcases/inference/conflicts_can_happen.dart
+++ b/pkg/front_end/testcases/inference/conflicts_can_happen.dart
@@ -22,12 +22,12 @@ class B {
 }
 
 class C1 implements A, B {
-  /*error:INVALID_METHOD_OVERRIDE*/ get a => null;
+  /*error:INVALID_METHOD_OVERRIDE*/ get /*@topType=dynamic*/ a => null;
 }
 
 // Still ambiguous
 class C2 implements B, A {
-  /*error:INVALID_METHOD_OVERRIDE*/ get a => null;
+  /*error:INVALID_METHOD_OVERRIDE*/ get /*@topType=dynamic*/ a => null;
 }
 
 main() {}