Do not duplicate constructors for named mixin applications.

lib/transformations/mixin_full_resolution.dart

Index: lib/transformations/mixin_full_resolution.dart
diff --git a/lib/transformations/mixin_full_resolution.dart b/lib/transformations/mixin_full_resolution.dart
index dfcc3c8174690aac765a9fd16a5c524a4bc50e48..f0c5cc38fcd4db15cb8c78172710cbd7c20ff7b2 100644
--- a/lib/transformations/mixin_full_resolution.dart
+++ b/lib/transformations/mixin_full_resolution.dart
@@ -95,13 +95,17 @@ class MixinFullResolution {
     }
     // For each generative constructor in the superclass we make a
     // corresponding forwarding constructor in the subclass.
-    var superclassSubstitution = getSubstitutionMap(class_.supertype);
-    var superclassCloner =
-        new CloneVisitor(typeSubstitution: superclassSubstitution);
-    for (var superclassConstructor in class_.superclass.constructors) {
-      var forwardingConstructor =
-          buildForwardingConstructor(superclassCloner, superclassConstructor);
-      class_.constructors.add(forwardingConstructor..parent = class_);
+    // Named mixin applications already have constructors, so only build the
+    // constructors for anonymous mixin applications.
+    if (class_.constructors.isEmpty) {
+      var superclassSubstitution = getSubstitutionMap(class_.supertype);
+      var superclassCloner =
+          new CloneVisitor(typeSubstitution: superclassSubstitution);
+      for (var superclassConstructor in class_.superclass.constructors) {
+        var forwardingConstructor =
+            buildForwardingConstructor(superclassCloner, superclassConstructor);
+        class_.constructors.add(forwardingConstructor..parent = class_);
+      }
     }
 
     // This class implements the mixin type.