Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 typedef F = () → core::int;
 abstract class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   abstract method x(() → core::int value) → void;
 }
 abstract class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     ;
   abstract method x(() → dynamic value) → void;
 }
 static method f<T extends core::Object>() → self::f::T
   ;
 static method g(self::B b) → dynamic
   ;
 static method main() → dynamic
   ;