One more minor mixin fix

We need to preserve the Mixin constructor to make sure metadata (e.g.,
implements) is injected in the right place.

R=jacobr@google.com

Review-Url: https://codereview.chromium.org/2779063002 .

pkg/dev_compiler/test/browser/language_tests.js

@@ -191,8 +191,6 @@ define(['dart_sdk', 'async_helper', 'expect', 'unittest', 'is', 'require'],
       'memory_swap_test': skip_timeout,
       'method_invocation_test': fail,
       'mint_arithmetic_test': fail,
-      'mixin_implements_test': fail,
-      'mixin_regress_13688_test': fail,
       'mixin_super_constructor_positionals_test_none_multi': fail, // Issue 28059
       'modulo_test': fail,
       'named_parameter_clash_test': fail,

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.
+  let 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);
   }
+  // Restore original Mixin constructor.
+  Mixin.prototype.constructor = constructor;  
   // Initializer methods: run mixin initializers, then the base.
   Mixin.prototype.new = function(...args) {
     // Run mixin initializers. They cannot have arguments.
@@ -55,6 +61,7 @@ mixin(base, @rest mixins) => JS(
         // Run base initializer.
         $base.prototype[namedCtor].apply(this, args);
       };
+      $defineNamedConstructor(Mixin, namedCtor);
     }
   }
 
@@ -626,6 +633,10 @@ defineNamedConstructorCallable(clazz, name, ctor) => JS(
   // Use defineProperty so we don't hit a property defined on Function,
   // like `caller` and `arguments`.
   $defineProperty($clazz, $name, { value: ctor, configurable: true });
+
+  let namedCtors = ${safeGetOwnProperty(clazz, _namedConstructors)};
+  if (namedCtors == null) $clazz[$_namedConstructors] = namedCtors = [];
+  namedCtors.push($name);
 })()''');
 
 defineEnumValues(enumClass, names) => JS(