Add ClosureClassMaps super interface for ClosureTask

pkg/compiler/lib/src/kernel/element_map_impl.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.
 
 library dart2js.kernel.element_map;
 
 import 'package:kernel/ast.dart' as ir;
 import 'package:kernel/clone.dart';
 import 'package:kernel/type_algebra.dart';
 
@@ skipping 667 matching lines @@
   }
 
   ResolutionImpact computeWorldImpact(KMember member) {
     return _memberList[member.memberIndex].getWorldImpact(this);
   }
 
   @override
   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.
 class _KEnv {
   final Set<ir.Program> programs = new Set<ir.Program>();
 
   Map<Uri, _KLibraryEnv> _libraryMap;
 
   /// TODO(johnniwinther): Handle arbitrary load order if needed.
   ir.Member get mainMethod => programs.first?.mainMethod;
@@ skipping 810 matching lines @@
     return node.isExternal &&
         !elementMap.isForeignLibrary(node.enclosingLibrary);
   }
 
   @override
   bool isJsInteropMember(MemberEntity element) {
     // TODO(johnniwinther): Compute this.
     return false;
   }
 }