Correctly minify named mixin applications in dart2dart.

sdk/lib/_internal/compiler/implementation/dart_backend/placeholder_collector.dart

Index: sdk/lib/_internal/compiler/implementation/dart_backend/placeholder_collector.dart
diff --git a/sdk/lib/_internal/compiler/implementation/dart_backend/placeholder_collector.dart b/sdk/lib/_internal/compiler/implementation/dart_backend/placeholder_collector.dart
index d184663b61af5504fffbe477b626ae13ff7b95f1..669aa02407bde8bfa5afbb9db2ace8e3a5d84cc3 100644
--- a/sdk/lib/_internal/compiler/implementation/dart_backend/placeholder_collector.dart
+++ b/sdk/lib/_internal/compiler/implementation/dart_backend/placeholder_collector.dart
@@ -579,6 +579,12 @@ class PlaceholderCollector extends Visitor {
     }
   }
 
+  visitNamedMixinApplication(NamedMixinApplication node) {
+    ClassElement classElement = currentElement;
+    makeElementPlaceholder(node.name, classElement);
+    node.visitChildren(this);
+  }
+
   bool tryResolveAndCollectTypeVariable(
       TypeDeclarationElement typeDeclaration, Identifier name) {
     // Hack for case when interface and default class are in different