Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_binary_custom.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::•()
     ;
   operator +(dynamic other) → core::int
     return 1;
   operator -(dynamic other) → core::double
     return 2.0;
 }
 static field core::int v_add = new self::A::•().{self::A::+}("foo");
 static field core::double v_minus = new self::A::•().{self::A::-}("bar");
 static method main() → dynamic {
   self::v_add;
   self::v_minus;
 }