Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_from_rhs_only_if_it_wont_conflict_with_overridden_fields2.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   final field dynamic x = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object implements self::A {
   final field dynamic x = 2;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method foo() → dynamic {
   core::String y = new self::B::•().{self::B::x};
   core::int z = new self::B::•().{self::B::x};
 }
 static method main() → dynamic {}