Add null check.

lib/compiler/implementation/tree/unparser.dart

Index: lib/compiler/implementation/tree/unparser.dart
diff --git a/lib/compiler/implementation/tree/unparser.dart b/lib/compiler/implementation/tree/unparser.dart
index b1ca8d66721bec5f2f2e90a71937cfe7b5e949e7..0e1179b9a28f3b6b0b5a8bee3a3c472a9257e0bb 100644
--- a/lib/compiler/implementation/tree/unparser.dart
+++ b/lib/compiler/implementation/tree/unparser.dart
@@ -90,8 +90,8 @@ class Unparser implements Visitor {
     // names are modelled with Send and it emits operator[] as only
     // operator, without [] which are expected to be emitted with
     // arguments.
-    Send send = node.name.asSend();
-    if (send !== null) {
+    if (node.name is Send) {
+      Send send = node.name;
       assert(send is !SendSet);
       visit(send.receiver);
       visit(send.selector);