Fix dart2dart and dart2js build.

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 a8672d0e17ccb1f24ea6e03ece4d7714a9016acc..c23ebed30acaf49ae70079d0953d7a8287c10afa 100644
--- a/lib/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/lib/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -314,6 +314,13 @@ class PlaceholderCollector extends AbstractVisitor {
   visitNode(Node node) { node.visitChildren(this); }  // We must go deeper.
 
   visitSend(Send send) {
+    Element element = treeElements[send];
+    if (element !== null && element.isErroneous()) {
+      // TODO(antonm): this is an unresolved constructor, not a dynamic send.
+      ErroneousElement erroneousElement = element;
+      compiler.cancel(reason: erroneousElement.errorMessage.toString(),
+                      node: send);
+    }
     new SendVisitor(this, treeElements).visitSend(send);
     send.visitChildren(this);
   }