One more minor mixin fix

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /// This library defines the operations that define and manipulate Dart
 /// classes.  Included in this are:
 ///   - Generics
 ///   - Class metadata
 ///   - Extension methods
 ///
@@ skipping 11 matching lines @@
 /// superclass (prototype).
 ///
 mixin(base, @rest mixins) => JS(
     '',
     '''(() => {
   // Create an initializer for the mixin, so when derived constructor calls
   // super, we can correctly initialize base and mixins.
 
   // Create a class that will hold all of the mixin methods.
   class Mixin extends $base {}
+  // Save the original constructor.  For ClassTypeAlias definitions, this
+  // is the concrete type.  We embed metadata (e.g., implemented interfaces)
+  // on this constructor and need to access that from runtime instances.
+  var constructor = Mixin.prototype.constructor;
   // Copy each mixin's methods, with later ones overwriting earlier entries.
   for (let m of $mixins) {
     $copyProperties(Mixin.prototype, m.prototype);

[Jennifer Messerly, 2017/03/28 20:47:48]

hmm, maybe just skip "constructor" when copying properties on this line, rather
than copy and copy back?

   }
+  // Restore original Mixin constructor.
+  Mixin.prototype.constructor = constructor;
   // Initializer method: run mixin initializers, then the base.
   Mixin.prototype.new = function(...args) {
     // Run mixin initializers. They cannot have arguments.
     // Run them backwards so most-derived mixin is initialized first.
     for (let i = $mixins.length - 1; i >= 0; i--) {
       $mixins[i].prototype.new.call(this);
     }
     // Run base initializer.
     $base.prototype.new.apply(this, args);
   };
@@ skipping 592 matching lines @@
     '''(() => {
   let values = [];
   for (var i = 0; i < $names.length; i++) {
     let value = $const_(new $enumClass(i));
     values.push(value);
     Object.defineProperty($enumClass, $names[i],
         { value: value, configurable: true });
   }
   $enumClass.values = $constList(values, $enumClass);
 })()''');