Add mixin support to source mirrors.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 1bb649129ed47366a3669aa841530b3756070598..7eb63715faf27781eacb3b88172581b694043a04 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -2172,12 +2172,14 @@ class MixinApplicationElementX extends BaseClassElementX
 
   Link<FunctionElement> constructors = new Link<FunctionElement>();
 
-  ClassElement mixin;
+  InterfaceType mixinType;
 
   MixinApplicationElementX(String name, Element enclosing, int id,
                            this.node, this.modifiers)
       : super(name, enclosing, id, STATE_NOT_STARTED);
 
+  ClassElement get mixin => mixinType != null ? mixinType.element : null;
+
   bool get isMixinApplication => true;
   bool get isUnnamedMixinApplication => node is! NamedMixinApplication;
   bool get hasConstructor => !constructors.isEmpty;