Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Bar extends self::Foo {
-  constructor •() → void
+  default constructor •() → void
     : super self::Foo::•()
     ;
 }
 class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method method() → self::Foo {
     return new self::Foo::•();
   }
 }
 class Sub extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
   method method() → self::Foo {
     return new self::Bar::•();
   }
 }
 static method main(core::List<core::String> args) → dynamic {
   dynamic object = args.length.==(0) ? new self::Base::•() : new self::Sub::•();
   dynamic a = object.method();
   core::print(a);
 }