Implement getInterfaceMember() for IncrementalClassHierarchy.

pkg/kernel/test/class_hierarchy_test.dart

Index: pkg/kernel/test/class_hierarchy_test.dart
diff --git a/pkg/kernel/test/class_hierarchy_test.dart b/pkg/kernel/test/class_hierarchy_test.dart
index 7da84c5a524bb16fbd7df4675351410deefbf1b6..45b187e72c3e923b95d4759618471d09db6965ef 100644
--- a/pkg/kernel/test/class_hierarchy_test.dart
+++ b/pkg/kernel/test/class_hierarchy_test.dart
@@ -46,16 +46,6 @@ class IncrementalClassHierarchyTest extends _ClassHierarchyTest {
   void test_forEachOverridePair_thisOverridesSupertype_setter() {
     super.test_forEachOverridePair_thisOverridesSupertype_setter();
   }
-
-  @failingTest
-  void test_getInterfaceMember_extends() {
-    super.test_getInterfaceMember_extends();
-  }
-
-  @failingTest
-  void test_getInterfaceMember_implements() {
-    super.test_getInterfaceMember_implements();
-  }
 }
 
 abstract class _ClassHierarchyTest {