Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Mixin extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method foo() → dynamic
     return 87;
 }
 abstract class Object&Mixin = core::Object with self::Mixin {
 }
 class Foo extends self::Object&Mixin {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method foo() → dynamic
     return super.{self::Mixin::foo}();
   method bar() → dynamic
     return super.{self::Mixin::field};
 }
 static method main() → dynamic {
   dynamic f = new self::Foo::•();
   f.field = 42;
   core::print(f.bar());
 }