dart2dart Collect type placeholder for default class.

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 9c4d614a767db57974c71055974545ef4d29bc0c..e4813c71903645dea9d6e8e612ea50e4205624be 100644
--- a/lib/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -414,6 +414,14 @@ class PlaceholderCollector extends AbstractVisitor {
     assert(currentElement is ClassElement);
     makeElementPlaceholder(node.name, currentElement);
     node.visitChildren(this);
+    if (node.defaultClause !== null) {
+      // Can't just visit class node's default clause because of the bug in the
+      // resolver, it just crashes when it meets type variable.
+      Type defaultType = (currentElement as ClassElement).defaultClass;
+      assert(defaultType !== null);
+      makeTypePlaceholder(node.defaultClause.typeName, defaultType);
+      visit(node.defaultClause.typeArguments);
+    }
   }
 
   visitTypedef(Typedef node) {