Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 class Base extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class Mixin extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method foo() → dynamic
     ;
 }
 abstract class Base&Mixin = self::Base with self::Mixin {
 }
 class Sub extends self::Base&Mixin {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic
   ;