Pass in `this` as a free variable to the closure class

pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart

Index: pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
index ae711e06405787400c276287c898e9028430f077..b1591635bb0adf4bd38c61f17880ab9ac6fc9338 100644
--- a/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/runtime_type_generator.dart
@@ -32,6 +32,7 @@ import '../js_backend/runtime_types.dart'
         TypeCheck,
         TypeChecks;
 import '../js_emitter/sorter.dart';
+import '../js_model/closure.dart' show JClosureField;
 import '../util/util.dart' show Setlet;
 import '../world.dart';
 
@@ -180,8 +181,10 @@ class RuntimeTypeGenerator {
       if (!method.isAbstract) {
         ScopeInfo scopeInfo = _closureDataLookup.getScopeInfo(method);
         if (scopeInfo is ClosureRepresentationInfo) {
-          ClosureFieldElement thisLocal = scopeInfo.thisFieldEntity;
+          FieldEntity thisLocal = scopeInfo.thisFieldEntity;
           if (thisLocal != null) {
+            assert(
+                thisLocal is ClosureFieldElement || thisLocal is JClosureField);
             jsAst.Name thisName = _namer.instanceFieldPropertyName(thisLocal);
             thisAccess = js('this.#', thisName);
           }