Set 'isSyntheticDefault' for default constructors.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method call() → core::int
     ;
 }
 class B extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   get call() → self::A
     ;
 }
 class D extends core::Object {
   field self::A fieldA;
   field self::B fieldB;
-  constructor •() → void
+  default constructor •() → void
     ;
   get getA() → self::A
     ;
   get getB() → self::B
     ;
 }
 static method main() → dynamic
   ;