Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/redirecting_factory.dart.outline.expect

Index: pkg/front_end/testcases/redirecting_factory.dart.outline.expect
diff --git a/pkg/front_end/testcases/redirecting_factory.dart.outline.expect b/pkg/front_end/testcases/redirecting_factory.dart.outline.expect
index 88ef2c77e1f1254635c7708e1ba55f6d0f88f04e..3f266d376fec0b90bc14875367c25df1c04359e1 100644
--- a/pkg/front_end/testcases/redirecting_factory.dart.outline.expect
+++ b/pkg/front_end/testcases/redirecting_factory.dart.outline.expect
@@ -19,7 +19,7 @@ class Bar<Sb extends core::Object, Tb extends core::Object> extends core::Object
     ;
 }
 class Builder<X extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method method() → dynamic
     ;
@@ -35,15 +35,15 @@ class SimpleCaseImpl<Ai extends core::Object, Bi extends core::Object> extends c
     let dynamic #redirecting_factory = self::SimpleCaseImpl2::• in invalid-expression;
 }
 class SimpleCaseImpl2<Ai2 extends core::Object, Bi2 extends core::Object> extends core::Object implements self::SimpleCaseImpl<self::SimpleCaseImpl2::Ai2, self::SimpleCaseImpl2::Bi2> {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Base<M extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Mixin<M extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Mix<M extends core::Object> = self::Base<self::Mix::M> with self::Mixin<self::Mix::M> {