Set 'isSyntheticDefault' for default constructors.

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