Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000048.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field core::bool v1;
   field core::num v2;
   constructor •(core::bool v1, core::num v2) → void
     : self::A::v1 = v1, self::A::v2 = v2, super core::Object::•()
     ;
 }
 class M1 extends core::Object {
   field core::num v2 = 0;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class C = self::A with self::M1 {
   constructor •(core::bool v1, core::num v2) → void
     : super self::A::•(v1, v2)
     ;
 }
 static method main() → dynamic {
   self::C c = new self::C::•(true, 2);
 }