Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.outline.expect
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.outline.expect b/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.outline.expect
index f5c0943651f2dedd4f318320170ea3398607904b..f93f8392afca6b8d5952db8671605c70a0039d16 100644
--- a/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.outline.expect
+++ b/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.outline.expect
@@ -3,11 +3,11 @@ import self as self;
 import "dart:core" as core;
 
 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
     ;
@@ -19,12 +19,12 @@ class B extends self::A {
     ;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Test extends core::Object {
   field self::B member;
-  constructor •() → void
+  default constructor •() → void
     ;
   method test() → void
     ;