Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/argument.dart.strong.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 abstract class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Foo extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
 }
 class Bar extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
 }
 class Baz extends self::Base {
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
 }
 static method foo(dynamic x) → void {}
 static method bar(dynamic x) → void {}
 static method foo_escaped(dynamic x) → void {}
 static method bar_escaped(dynamic x) → void {}
 static method escape(dynamic fn) → void {
   fn.call(new self::Baz::•());
 }
 static method main() → dynamic {
   self::foo(new self::Foo::•());
   self::bar(new self::Bar::•());
   self::escape(self::foo_escaped);
   self::escape(self::bar_escaped);
 }