Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B<T extends core::Object> extends self::A<self::B::T> {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
 }
 static method main() → dynamic {
   core::num x;
   core::int x1 = x = 1;
   core::double x2 = x = 1.0;
 }