Implement optimized mixin application in dart2js

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index e282c0be957c2c6ce5ba1396913f0ed3a80fa11f..58672556603457beb38e8bd19d19649b158e24e8 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -1074,6 +1074,9 @@ class LibraryElementX extends ElementX
   final Map<LibraryDependency, LibraryElement> tagMapping =
       new Map<LibraryDependency, LibraryElement>();
 
+  final Map<String, MixinApplicationElementX> mixinApplicationCache =
+      <String, MixinApplicationElementX>{};
+
   LibraryElementX(Script script, [Uri canonicalUri, LibraryElementX origin])
       : this.canonicalUri =
             ((canonicalUri == null) ? script.readableUri : canonicalUri),