Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/prefer_baseclass.dart.strong.expect

Index: pkg/front_end/testcases/prefer_baseclass.dart.strong.expect
diff --git a/pkg/front_end/testcases/prefer_baseclass.dart.strong.expect b/pkg/front_end/testcases/prefer_baseclass.dart.strong.expect
index 4783efb3bf105b25fb53fbbb72491ea9eda0a70f..d02c817be4130871847cb7f0df466e9df10613fd 100644
--- a/pkg/front_end/testcases/prefer_baseclass.dart.strong.expect
+++ b/pkg/front_end/testcases/prefer_baseclass.dart.strong.expect
@@ -3,32 +3,32 @@ import self as self;
 import "dart:core" as core;
 
 abstract class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 abstract class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class AB1 extends self::A implements self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }
 class AB2 extends self::A implements self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }
 class BA1 extends self::B implements self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }
 class BA2 extends self::B implements self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }