Add ClosureClassMaps super interface for ClosureTask

pkg/compiler/lib/src/kernel/element_map_impl.dart

Index: pkg/compiler/lib/src/kernel/element_map_impl.dart
diff --git a/pkg/compiler/lib/src/kernel/element_map_impl.dart b/pkg/compiler/lib/src/kernel/element_map_impl.dart
index 1bf6220f785493cdb51032da9628b41826cf7e98..c2f0d116e2b0ca8b7c4c133ca96c7f0cd28b9222 100644
--- a/pkg/compiler/lib/src/kernel/element_map_impl.dart
+++ b/pkg/compiler/lib/src/kernel/element_map_impl.dart
@@ -685,6 +685,11 @@ class KernelToElementMapImpl extends KernelToElementMapMixin {
   Spannable getSpannable(MemberEntity member, ir.Node node) {
     return member;
   }
+
+  /// Returns the kernel IR node that defines the [member].
+  ir.Member getMemberNode(KMember member) {
+    return _memberList[member.memberIndex].node;
+  }
 }
 
 /// Environment for fast lookup of program libraries.