Use closure conversion in new dart2js backend.

pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.dart b/pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.dart
index 0e3c61aa4bb0eb5b61aa9aa434e155572ac4c9b6..a2828b697d175d31114ec71987939288743b0405 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/copy_propagator.dart
@@ -224,6 +224,13 @@ class CopyPropagator extends RecursiveVisitor with PassMixin {
     return node;
   }
 
+  Statement visitSetField(SetField node) {
+    node.next = visitStatement(node.next);
+    visitExpression(node.value);
+    visitExpression(node.object);
+    return node;
+  }
+
   void visitFunctionExpression(FunctionExpression node) {
     new CopyPropagator().rewrite(node.definition);
   }