Traverse newly discovered elements to pick more referenced types.

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 fadb9404dc032a8337f6ceef1397a90b280eaa63..76a8ff03e693cb1f8be2478a84d4f33e137cfa3f 100644
--- a/lib/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -145,6 +145,9 @@ class PlaceholderCollector extends AbstractVisitor {
       currentElement = element.variables;
       elementNode = currentElement.parseNode(compiler);
       collectFieldDeclarationPlaceholders(element, elementNode);
+    } else if (element is ClassElement || element is TypedefElement) {
+      currentElement = element;
+      elementNode = currentElement.parseNode(compiler);
     } else {
       assert(false); // Unreachable.
     }
@@ -192,10 +195,6 @@ class PlaceholderCollector extends AbstractVisitor {
 
   visitNode(Node node) { node.visitChildren(this); }  // We must go deeper.
 
-  visitClassNode(ClassNode node) {
-    internalError('Should never meet ClassNode', node);
-  }
-
   visitSend(Send send) {
     new SendVisitor(this, treeElements).visitSend(send);
     send.visitChildren(this);