Compute correct constructor type for synthesized mixin constructors.

sdk/lib/_internal/compiler/implementation/typechecker.dart

Index: sdk/lib/_internal/compiler/implementation/typechecker.dart
diff --git a/sdk/lib/_internal/compiler/implementation/typechecker.dart b/sdk/lib/_internal/compiler/implementation/typechecker.dart
index 5dd026aa6ed882f957bd4466555d31f034784265..ff499fc4548274054087a5d8e703feb0980490af 100644
--- a/sdk/lib/_internal/compiler/implementation/typechecker.dart
+++ b/sdk/lib/_internal/compiler/implementation/typechecker.dart
@@ -1334,10 +1334,19 @@ class TypeCheckerVisitor extends Visitor<DartType> {
     if (Elements.isUnresolved(constructor)) return types.dynamicType;
     DartType constructorType = constructor.computeType(compiler);
     if (identical(type.kind, TypeKind.INTERFACE)) {
-      InterfaceType interfaceType = type;
-      constructorType = constructorType.subst(
-          interfaceType.typeArguments,
-          interfaceType.element.typeVariables);
+      if (constructor.isSynthesized) {
+        // TODO(johnniwinther): Remove this when synthesized constructors handle
+        // type variables correctly.
+        InterfaceType interfaceType = type;
+        ClassElement receiverElement = interfaceType.element;
+        while (receiverElement.isMixinApplication) {
+          receiverElement = receiverElement.supertype.element;
+        }
+        constructorType = constructorType.substByContext(
+            interfaceType.asInstanceOf(receiverElement));
+      } else {
+        constructorType = constructorType.substByContext(type);
+      }
     }
     return constructorType;
   }