Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/super_mixin.dart.strong.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "./mixin_library.dart" as mix;
 
 class Super<S extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method foo() → dynamic
     return 40;
   method f() → dynamic
     return 3;
 }
 abstract class Super&Mixin^#T0^#T0<#T0 extends core::Object> = self::Super<self::Super&Mixin^#T0^#T0::#T0> with mix::Mixin<self::Super&Mixin^#T0^#T0::#T0> {
 }
 class C<V extends core::Object> extends self::Super&Mixin^#T0^#T0<self::C::V> {
-  constructor •() → void
+  default constructor •() → void
     : super self::Super::•()
     ;
 }
 abstract class Super&Mixin = self::Super<dynamic> with mix::Mixin<dynamic> {
 }
 class D extends self::Super&Mixin {
-  constructor •() → void
+  default constructor •() → void
     : super self::Super::•()
     ;
 }
 class C2<V extends core::Object> = self::Super<self::C2::V> with mix::Mixin<self::C2::V> {
   constructor •() → void
     : super self::Super::•()
     ;
 }
 class D2 = self::Super<dynamic> with mix::Mixin<dynamic> {
   constructor •() → void
     : super self::Super::•()
     ;
 }
 static method main() → dynamic {
   core::print(new self::C::•<dynamic>().{mix::Mixin::foo}());
   core::print(new self::C2::•<dynamic>().{mix::Mixin::foo}());
 }