Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/try_catch.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 core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class E extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method test(() → void f) → void {
   try {
     core::int x = 0;
     f.call();
   }
   on self::C catch(no-exception-var) {
     core::int x = 0;
   }
   on self::D catch(final self::D x) {
     self::D x2 = x;
   }
   on self::E catch(final self::E x, final core::StackTrace y) {
     self::E x2 = x;
     core::StackTrace y2 = y;
   }
   on dynamic catch(final dynamic x, final core::StackTrace y) {
     dynamic x2 = x;
     core::StackTrace y2 = y;
   }
 }
 static method main() → dynamic {}