Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_assign_to_property_full.dart.direct.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::•()
     ;
 }
 class Test extends core::Object {
   field self::B member = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   static method test(self::Test t) → void {
     t.member = self::f<dynamic>();
     let final dynamic #t1 = t in #t1.member.==(null) ? #t1.member = self::f<dynamic>() : null;
     let final dynamic #t2 = t in #t2.member = #t2.member.+(self::f<dynamic>());
     let final dynamic #t3 = t in #t3.member = #t3.member.*(self::f<dynamic>());
     let final dynamic #t4 = t in #t4.member = #t4.member.&(self::f<dynamic>());
     let final dynamic #t5 = t in #t5.member = #t5.member.-(1);
     let final dynamic #t6 = t in #t6.member = #t6.member.-(1);
     dynamic v1 = t.member = self::f<dynamic>();
     dynamic v2 = let final dynamic #t7 = t in let final dynamic #t8 = #t7.member in #t8.==(null) ? #t7.member = self::f<dynamic>() : #t8;
     dynamic v3 = let final dynamic #t9 = t in #t9.member = #t9.member.+(self::f<dynamic>());
     dynamic v4 = let final dynamic #t10 = t in #t10.member = #t10.member.*(self::f<dynamic>());
     dynamic v5 = let final dynamic #t11 = t in #t11.member = #t11.member.&(self::f<dynamic>());
     dynamic v6 = let final dynamic #t12 = t in #t12.member = #t12.member.-(1);
     dynamic v7 = let final dynamic #t13 = t in let final dynamic #t14 = #t13.member in let final dynamic #t15 = #t13.member = #t14.-(1) in #t14;
   }
 }
 static method f<T extends core::Object>() → self::f::T
   return null;
 static method main() → dynamic {}