Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class C1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class C2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class C3 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class A<T extends core::Object> extends core::Object {
   static field dynamic _redirecting# = <dynamic>[self::A::a, self::A::b, self::A::c];
   constructor internal() → void
     ;
   static factory a<T extends core::Object>() → self::A<self::A::a::T>
     let dynamic #redirecting_factory = self::B::a in invalid-expression;
   static factory b<T extends core::Object>() → self::A<self::A::b::T>
     let dynamic #redirecting_factory = self::B::a in invalid-expression;
   static factory c<T extends core::Object>() → self::A<self::A::c::T>
     let dynamic #redirecting_factory = "Missing" in invalid-expression;
 }
 class B<S extends core::Object> extends self::A<self::B::S> {
   static field dynamic _redirecting# = <dynamic>[self::B::a, self::B::b];
   constructor internal() → void
     ;
   static factory a<S extends core::Object>() → self::B<self::B::a::S>
     let dynamic #redirecting_factory = self::C::• in invalid-expression;
   static factory b<S extends core::Object>() → self::B<self::B::b::S>
     let dynamic #redirecting_factory = self::C::• in invalid-expression;
 }
 class C<U extends core::Object> extends self::B<self::C::U> {
   constructor •() → void
     ;
 }
 static method main() → dynamic
   ;