Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class E extends core::Object {
-  constructor •() → self::E
+  default constructor •() → self::E
     : super core::Object::•();
   method foo() → dynamic {
     super.[]=(4, 42);
     let final dynamic #t1 = 4 in super.[]=(#t1, super.[](#t1).+(5));
     return super.[](2);
   }
 }
 static method beforeTestMissingTry() → dynamic {
   throw core::_genericNoSuchMethod(null, #testMissingTry, <dynamic>[], <dynamic, dynamic>{}, null);
 }
 static method testMissingTry() → dynamic
   invalid-statement;
 static method main() → dynamic {}