Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_assign_to_property_null_aware.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::•()
     ;
 }
 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 {
     let final dynamic #t1 = t in #t1.==(null) ? null : #t1.{self::Test::member} = self::f<self::B>();
     let final dynamic #t2 = t in #t2.==(null) ? null : #t2.{self::Test::member}.{core::Object::==}(null) ? #t2.{self::Test::member} = self::f<self::B>() : null;
     let final dynamic #t3 = t in #t3.==(null) ? null : #t3.{self::Test::member} = #t3.{self::Test::member}.{self::B::+}(self::f<dynamic>());
     let final dynamic #t4 = t in #t4.==(null) ? null : #t4.{self::Test::member} = #t4.{self::Test::member}.{self::B::*}(self::f<dynamic>());
     let final dynamic #t5 = t in #t5.==(null) ? null : #t5.{self::Test::member} = #t5.{self::Test::member}.{self::B::&}(self::f<dynamic>());
     let final dynamic #t6 = t in #t6.==(null) ? null : #t6.{self::Test::member} = #t6.{self::Test::member}.{self::B::-}(1);
     let final dynamic #t7 = t in #t7.==(null) ? null : #t7.{self::Test::member} = #t7.{self::Test::member}.{self::B::-}(1);
     self::B v1 = let final dynamic #t8 = t in #t8.==(null) ? null : #t8.{self::Test::member} = self::f<self::B>();
     self::B v2 = let final dynamic #t9 = t in #t9.==(null) ? null : let final dynamic #t10 = #t9.{self::Test::member} in #t10.{core::Object::==}(null) ? #t9.{self::Test::member} = self::f<self::B>() : #t10;
     self::A v3 = let final dynamic #t11 = t in #t11.==(null) ? null : #t11.{self::Test::member} = #t11.{self::Test::member}.{self::B::+}(self::f<dynamic>());
     self::B v4 = let final dynamic #t12 = t in #t12.==(null) ? null : #t12.{self::Test::member} = #t12.{self::Test::member}.{self::B::*}(self::f<dynamic>());
     self::C v5 = let final dynamic #t13 = t in #t13.==(null) ? null : #t13.{self::Test::member} = #t13.{self::Test::member}.{self::B::&}(self::f<dynamic>());
     self::B v6 = let final dynamic #t14 = t in #t14.==(null) ? null : #t14.{self::Test::member} = #t14.{self::Test::member}.{self::B::-}(1);
     self::B v7 = let final dynamic #t15 = t in #t15.==(null) ? null : let final dynamic #t16 = #t15.{self::Test::member} in let final dynamic #t17 = #t15.{self::Test::member} = #t16.{self::B::-}(1) in #t16;
   }
 }
 static method f<T extends core::Object>() → self::f::T
   return null;
 static method main() → dynamic {}