Remove some warnings and dead code in dart2js.

lib/compiler/implementation/dart_backend/placeholder_collector.dart

Index: lib/compiler/implementation/dart_backend/placeholder_collector.dart
diff --git a/lib/compiler/implementation/dart_backend/placeholder_collector.dart b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
index f3753032538b3561a5ae934db9533736fa0ae914..d6bbd20682073f6808ac2c9830fa579ad3ce3603 100644
--- a/lib/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -475,9 +475,10 @@ class PlaceholderCollector extends Visitor {
       } else {
         if (hasPrefix) {
           assert(node.typeName is Send);
-          assert(node.typeName.receiver is Identifier);
-          assert(node.typeName.selector is Identifier);
-          makeNullPlaceholder(node.typeName.receiver);
+          Send typeName = node.typeName;
+          assert(typeName.receiver is Identifier);
+          assert(typeName.selector is Identifier);
+          makeNullPlaceholder(typeName.receiver);
         }
         if (typeElement !== dynamicTypeElement) {
           makeTypePlaceholder(target, type);