Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_type_on_overridden_fields2.dart.strong.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field core::int x = 2;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
   get x() → core::int
     return 3;
 }
 static method foo() → dynamic {
   core::String y = new self::B::•().{self::B::x};
   core::int z = new self::B::•().{self::B::x};
 }
 static method main() → dynamic {
   self::foo();
 }