Handle platform access through prefix in dart2dart

pkg/compiler/lib/src/dart_backend/placeholder_collector.dart

Index: pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
diff --git a/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart b/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
index c4031193d21ba30d6727bf34c42f21574872a1f4..60f3fe87ec9caa51a321414c2b71b5b0d3e24d6a 100644
--- a/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
+++ b/pkg/compiler/lib/src/dart_backend/placeholder_collector.dart
@@ -279,7 +279,7 @@ class PlaceholderCollector extends Visitor {
   void tryMakeMemberPlaceholder(Identifier node) {
     assert(node != null);
     if (node is Operator) return;
-    final identifier = node.source;
+    String identifier = node.source;
     if (fixedMemberNames.contains(identifier)) return;
     memberPlaceholders.putIfAbsent(
         identifier, () => new Set<Identifier>()).add(node);