Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/null_aware_method_invocation.dart.direct.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method f() → core::int
     return null;
 }
 static method g(self::C c) → dynamic {
   dynamic x = let final dynamic #t1 = c in #t1.==(null) ? null : #t1.f();
   let final dynamic #t2 = c in #t2.==(null) ? null : #t2.f();
 }
 static method main() → dynamic {}