Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/inference/infer_assign_to_implicit_this_upwards.dart.outline.expect

 library test;
 import self as self;
 import "dart:core" as core;
 
 class Test1 extends core::Object {
   field core::int t;
-  constructor •() → void
+  default constructor •() → void
     ;
   method test() → void
     ;
 }
 class Test2 extends core::Object {
   field core::num t;
-  constructor •() → void
+  default constructor •() → void
     ;
   method test() → void
     ;
 }
 class Test3 extends core::Object {
   field core::double t;
-  constructor •() → void
+  default constructor •() → void
     ;
   method test3() → void
     ;
 }
 static method getInt() → core::int
   ;
 static method getNum() → core::num
   ;
 static method getDouble() → core::double
   ;
 static method main() → dynamic
   ;