Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect

Index: pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
index 1c4d84df864f2b3c879d48a78cb83c40401218d9..2f9029cb9c8870db7c4a28ded3d5dcd7e05de6f8 100644
--- a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
@@ -4,7 +4,7 @@ import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 abstract class Object&Mixin = core::Object with self::Mixin {
@@ -12,6 +12,6 @@ abstract class Object&Mixin = core::Object with self::Mixin {
 abstract class Object&Mixin&Mixin = self::Object&Mixin with self::Mixin {
 }
 class A extends self::Object&Mixin&Mixin {
-  constructor •() → void
+  default constructor •() → void
     ;
 }