Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000081.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Base extends core::Object {
   field core::int hashCode = 42;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class Sub extends self::Base {
   field core::int _hashCode = null;
-  constructor •() → void
+  default constructor •() → void
     : super self::Base::•()
     ;
   get hashCode() → dynamic
     return let final dynamic #t1 = this._hashCode in #t1.==(null) ? this._hashCode = this.{=self::Base::hashCode} : #t1;
   method foo() → dynamic {
     this._hashCode.==(null) ? this._hashCode = this.{=self::Base::hashCode} : null;
   }
 }
 static method main() → dynamic {
   core::print(new self::Sub::•().hashCode);
   dynamic l = <dynamic>[null];
   let final dynamic #t2 = l in let final dynamic #t3 = 0 in #t2.[](#t3).==(null) ? #t2.[]=(#t3, "fisk") : null;
 }