Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     : super self::A::•()
     ;
   operator +(self::C v) → self::A
     return null;
   operator -(core::int i) → self::B
     return null;
   operator *(self::B v) → self::B
     return null;
   operator &(self::A v) → self::C
     return null;
 }
 class C extends self::B {
-  constructor •() → void
+  default constructor •() → void
     : super self::B::•()
     ;
 }
 static method f<T extends core::Object>() → self::f::T
   return null;
 static method test() → void {
   self::B local;
   local = self::f<self::B>();
   local.{core::Object::==}(null) ? local = self::f<self::B>() : null;
   local = local.{self::B::+}(self::f<dynamic>());
   local = local.{self::B::*}(self::f<dynamic>());
   local = local.{self::B::&}(self::f<dynamic>());
   local = local.{self::B::-}(1);
   local = local.{self::B::-}(1);
   self::B v1 = local = self::f<self::B>();
   self::B v2 = let final dynamic #t1 = local in #t1.{core::Object::==}(null) ? local = self::f<self::B>() : #t1;
   self::A v3 = local = local.{self::B::+}(self::f<dynamic>());
   self::B v4 = local = local.{self::B::*}(self::f<dynamic>());
   self::C v5 = local = local.{self::B::&}(self::f<dynamic>());
   self::B v6 = local = local.{self::B::-}(1);
   self::B v7 = let final dynamic #t2 = local in let final dynamic #t3 = local = #t2.{self::B::-}(1) in #t2;
 }
 static method main() → dynamic {}