Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000070.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 
 class A<N extends core::Object, S extends core::Object, U extends core::Object> extends core::Object {
   final field core::List<self::A::U> field;
   constructor •(self::A::N n, self::A::S s) → void
     : self::A::field = core::List::_internal<self::A::U>(), super core::Object::•() {
     exp::Expect::isTrue(n is self::A::N);
     exp::Expect::isTrue(s is self::A::S);
   }
   constructor empty() → void
     : self::A::field = null, super core::Object::•() {}
   const constructor c(self::A::U u, self::A::S s) → void
     : self::A::field = const <dynamic>[null], super core::Object::•()
     ;
   static factory f<N extends core::Object, S extends core::Object, U extends core::Object>(self::A::f::S s) → self::A<self::A::f::N, self::A::f::S, self::A::f::U> {
     exp::Expect::isTrue(s is self::A::f::S);
     return new self::A::empty<dynamic, dynamic, dynamic>();
   }
   get getter() → core::List<self::A::U> {
     return this.field;
   }
   set setter(self::A::S s) → void {}
 }
 abstract class J<Aa extends core::Object, B extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 abstract class I<H extends core::Object, C extends core::Object, K extends core::Object> extends self::J<self::I::C, self::I::K> {
-  constructor •() → void
+  default constructor •() → void
     : super self::J::•()
     ;
 }
 static method main() → dynamic {
   new self::A::•<core::num, core::double, core::List<dynamic>>(1, 2.0);
   self::A<dynamic, dynamic, dynamic> a = self::A::f<core::int, core::int, core::int>(1);
   const self::A::c<core::int, core::int, core::List<dynamic>>(const <dynamic>[], 1);
   dynamic z = a.getter;
   a.setter = 1;
 }