Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000067.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 
 class A extends core::Object {
   static field dynamic _redirecting# = <dynamic>[self::A::foo];
   constructor •() → void
     : super core::Object::•() {}
   static factory foo() → self::A
     let dynamic #redirecting_factory = self::C::bar in invalid-expression;
   method m() → core::int {}
 }
 class C extends self::A {
   static field dynamic _redirecting# = <dynamic>[self::C::bar];
   constructor •() → void
     : super self::A::•() {}
   static factory bar() → self::C
     let dynamic #redirecting_factory = self::D::• in invalid-expression;
   method m() → core::int {
     return 1;
   }
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     : super self::C::•()
     ;
   method m() → core::int {
     return 2;
   }
 }
 static method main() → dynamic {
   self::A a = new self::D::•();
   exp::Expect::equals(2, a.m());
 }