Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/call.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Callable extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method call(dynamic x) → dynamic {
     return "string";
   }
 }
 class CallableGetter extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get call() → dynamic
     return new self::Callable::•();
 }
 static method main() → dynamic {
   dynamic closure = (dynamic x) → dynamic => x;
   dynamic int1 = closure.call(1);
   dynamic int2 = closure.call(1);
   dynamic int3 = closure.call.call(1);
@@ skipping 14 matching lines @@
   dynamic nothing4 = closure.call.call.call();
   dynamic nothing5 = callable.call();
   dynamic nothing6 = callable.call();
   dynamic nothing7 = callable.call.call();
   dynamic nothing8 = callable.call.call.call();
   dynamic nothing9 = callableGetter.call();
   dynamic nothing10 = callableGetter.call();
   dynamic nothing11 = callableGetter.call.call();
   dynamic nothing12 = callableGetter.call.call.call();
 }