Set 'isSyntheticDefault' for default constructors.

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

Index: pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.strong.expect b/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.strong.expect
index a95b282716087512f663b133db09cffe2cc0a8a1..03375144ecaa53b1c2e5219f7bf600896d34c4f9 100644
--- a/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/infer_assign_to_implicit_this.dart.strong.expect
@@ -3,12 +3,12 @@ import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
   operator +(self::C v) → self::A
@@ -21,13 +21,13 @@ class B extends self::A {
     return null;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }
 class Test extends core::Object {
   field self::B member = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method test() → void {