Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/optional.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::•()
     ;
-  method method(dynamic x, [dynamic y, dynamic z]) → dynamic {
+  method method(dynamic x, [dynamic y = null, dynamic z = null]) → dynamic {
     return "string";
   }
 }
 abstract class External extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
-  abstract method externalMethod(core::int x, [core::int y, core::int z]) → core::String;
+  abstract method externalMethod(core::int x, [core::int y = null, core::int z = null]) → core::String;
   abstract method listen(self::Listener listener) → void;
 }
 abstract class Listener extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
-  abstract method event(core::String input, [core::int x, core::int y]) → void;
+  abstract method event(core::String input, [core::int x = null, core::int y = null]) → void;
 }
 class TestListener extends self::Listener {
-  constructor •() → void
+  default constructor •() → void
     : super self::Listener::•()
     ;
-  method event(dynamic input, [dynamic x, dynamic y]) → void {}
+  method event(dynamic input, [dynamic x = null, dynamic y = null]) → void {}
 }
 class ExtendedListener extends self::Listener {
-  constructor •() → void
+  default constructor •() → void
     : super self::Listener::•()
     ;
-  method event(dynamic input, [dynamic x, dynamic y, dynamic z]) → void {}
+  method event(dynamic input, [dynamic x = null, dynamic y = null, dynamic z = null]) → void {}
 }
 class InvalidListener extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
-  method event(dynamic input, [dynamic x]) → void {}
+  method event(dynamic input, [dynamic x = null]) → void {}
 }
-external static method createExternal() → self::External;
+external static abstract method createExternal() → self::External;
 static method main() → dynamic {
   dynamic foo = new self::Foo::•();
   dynamic string1 = foo.method(1);
   dynamic string2 = foo.method(1, 2);
   dynamic string3 = foo.method(1, 2, 3);
   dynamic extern = self::createExternal();
   dynamic string4 = extern.externalMethod(1);
   dynamic string5 = extern.externalMethod(1, 2);
   dynamic string6 = extern.externalMethod(1, 2, 3);
   extern.listen(new self::TestListener::•());
   extern.listen(new self::ExtendedListener::•());
   extern.listen(new self::InvalidListener::•());
   dynamic nothing1 = foo.method();
   dynamic nothing2 = foo.method(1, 2, 3, 4);
   dynamic nothing3 = extern.externalMethod();
   dynamic nothing4 = extern.externalMethod(1, 2, 3, 4);
 }