Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/store_load.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
   field dynamic _field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class FooValue extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Bar extends core::Object {
   field dynamic _field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class BarValue extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method main() → dynamic {
   dynamic foo = new self::Foo::•();
   foo._field = new self::FooValue::•();
   dynamic fooValue = foo._field;
   core::print(fooValue);
   dynamic bar = new self::Bar::•();
   bar._field = new self::BarValue::•();
   dynamic barValue = bar._field;
   core::print(barValue);
 }