Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/inferred_type_custom_binary_op.dart.strong.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::•()
     ;
   operator *(self::C other) → core::bool
     return true;
 }
 static field self::C c = new self::C::•();
 static field core::bool x = self::c.{self::C::*}(self::c);
 static method main() → dynamic {
   self::c;
   self::x;
 }