Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.outline.expect

Index: pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.outline.expect
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.outline.expect b/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.outline.expect
index 4394e22ef63cae1c1866e1485444a595570413d2..3d9f3a4de477707a2978cd3df56ada1156436a8f 100644
--- a/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.outline.expect
+++ b/pkg/front_end/testcases/inference/infer_assign_to_index_this.dart.outline.expect
@@ -3,15 +3,15 @@ import self as self;
 import "dart:core" as core;
 
 class Index extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     ;
   operator +(self::C v) → self::A
     ;
@@ -23,11 +23,11 @@ class B extends self::A {
     ;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Test extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   operator [](self::Index i) → self::B
     ;