Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/try_catch_promotion.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::•()
     ;
 }
 class D extends self::C {
-  constructor •() → void
+  default constructor •() → void
     : super self::C::•()
     ;
 }
 class E extends core::StackTrace {
-  constructor •() → void
+  default constructor •() → void
     : super core::StackTrace::•()
     ;
 }
 static method test(() → void f) → void {
   try {
     f.call();
   }
   on self::C catch(final self::C x, final core::StackTrace y) {
     self::C x1 = x;
     core::StackTrace y1 = y;
     if(x is self::D) {
       self::D x2 = x{self::D};
     }
     if(y is self::E) {
       self::E y2 = y{self::E};
     }
   }
 }
 static method main() → dynamic {}