One more minor mixin fix

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

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
index a0f02c7ac40553dacd76821115e80be28454f948..ef561b18fbdaaee0f28f59cf977d98403c07cc9c 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -29,10 +29,16 @@ mixin(base, @rest mixins) => JS(
 
   // 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.