Make the ClosedWorld build the closure class on the kernel side.

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

Index: pkg/compiler/lib/src/kernel/element_map.dart
diff --git a/pkg/compiler/lib/src/kernel/element_map.dart b/pkg/compiler/lib/src/kernel/element_map.dart
index f75239372cfde791079e14d24702072481ecd17a..dfa6ae18ec7d54f3d1aecd30be165109367538d8 100644
--- a/pkg/compiler/lib/src/kernel/element_map.dart
+++ b/pkg/compiler/lib/src/kernel/element_map.dart
@@ -210,6 +210,9 @@ enum MemberKind {
   // A closure class `call` method whose body is defined by an
   // [ir.FunctionExpression].
   closureCall,
+  // A field corresponding to a captured variable in the closure. It does not
+  // have a corresponding ir.Node.
+  closureField,
 }
 
 /// Definition information for a [MemberEntity].