Set 'isSyntheticDefault' for default constructors.

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

 library;
 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::•()
     ;
 }
 static method takeSubclassOfA(dynamic obj) → dynamic {}
 static method takeSubclassOfB(dynamic obj) → dynamic {}
 static method main() → dynamic {
   self::takeSubclassOfA(new self::AB1::•());
   self::takeSubclassOfA(new self::AB2::•());
   self::takeSubclassOfB(new self::BA1::•());
   self::takeSubclassOfB(new self::BA2::•());
 }