Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/call_corner_cases.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::•()
     ;
   method call() → core::int
     return 0;
 }
 class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get call() → self::A
     return new self::A::•();
 }
 class D extends core::Object {
   field self::A fieldA = new self::A::•();
   field self::B fieldB = new self::B::•();
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   get getA() → self::A
     return new self::A::•();
   get getB() → self::B
     return new self::B::•();
 }
 static method main() → dynamic {
   core::int callA = new self::A::•().{self::A::call}();
   dynamic callB = new self::B::•().{self::B::call}();
   core::int callFieldA = new self::D::•().{self::D::fieldA}();
   core::int callGetA = new self::D::•().{self::D::getA}();
   dynamic callFieldB = new self::D::•().{self::D::fieldB}();
   dynamic callGetB = new self::D::•().{self::D::getB}();
 }