Add more mixin tests and start rejecting illegal syntax.

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 f8ea3d248da961363c7bbee4b46a01f46649cda2..59ae89c4ee0ee2e2f1a0bf6a67e7d260ecc7d24b 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -1781,6 +1781,7 @@ abstract class ClassElementX extends BaseClassElementX {
 class MixinApplicationElementX extends BaseClassElementX
     implements MixinApplicationElement {
   final Node node;
+  final Modifiers modifiers;
 
   FunctionElement constructor;
   ClassElement mixin;
@@ -1791,7 +1792,7 @@ class MixinApplicationElementX extends BaseClassElementX
   final ClassElement origin = null;
 
   MixinApplicationElementX(SourceString name, Element enclosing, int id,
-                           this.node)
+                           this.node, this.modifiers)
       : super(name, enclosing, id, STATE_NOT_STARTED);
 
   bool get isMixinApplication => true;