Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C<T extends core::num> extends core::Object {
   field self::C::T a = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method op(self::C::T b) → void {
     self::C::T r1 = this.{self::C::a}.{core::num::+}(b);
     self::C::T r2 = this.{self::C::a}.{core::num::-}(b);
     self::C::T r3 = this.{self::C::a}.{core::num::*}(b);
   }
   method opEq(self::C::T b) → void {
     this.{self::C::a} = this.{self::C::a}.{core::num::+}(b);
     this.{self::C::a} = this.{self::C::a}.{core::num::-}(b);
     this.{self::C::a} = this.{self::C::a}.{core::num::*}(b);
   }
 }
 static method main() → dynamic {}