Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/mixin.dart.outline.expect

 library;
 import self as self;
 import "dart:core" as core;
 
 abstract class Object&M1 = core::Object with self::M1 {
 }
 abstract class Object&M1&M2 = self::Object&M1 with self::M2 {
 }
 class B extends self::Object&M1&M2 {
   constructor •(dynamic value) → void
     ;
 }
 abstract class M1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method m() → dynamic
     ;
 }
 abstract class M2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method m() → dynamic
     ;
 }
 class C extends self::Object&M1&M2 {
   constructor •(dynamic value) → void
     ;
 }
 abstract class G1<T extends core::Object> extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method m() → dynamic
     ;
 }
 abstract class Object&G1^^#T0<#T0 extends core::Object> = core::Object with self::G1<self::Object&G1^^#T0::#T0> {
 }
 class D<S extends core::Object> extends self::Object&G1^^#T0<self::D::S> {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 static method main() → dynamic
   ;