Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/super_operator.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   operator +(core::String s) → dynamic
     ;
   operator [](dynamic i) → dynamic
     ;
   operator []=(dynamic i, dynamic val) → dynamic
     ;
 }
 class B extends self::A {
-  constructor •() → void
+  default constructor •() → void
     ;
   operator +(core::String s) → dynamic
     ;
   operator [](dynamic i) → dynamic
     ;
   operator []=(dynamic i, dynamic val) → dynamic
     ;
 }
 class Autobianchi extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method g() → dynamic
     ;
 }