Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method instanceMethod() → dynamic
     ;
 }
 abstract class ExternalValue extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 abstract class Bar extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   abstract method externalInstanceMethod() → self::ExternalValue;
 }
 class Box extends core::Object {
   field dynamic field;
-  constructor •() → void
+  default constructor •() → void
     ;
 }
 class FinalBox extends core::Object {
   final field dynamic finalField;
   constructor •(dynamic finalField) → void
     ;
 }
 class SubFinalBox extends self::FinalBox {
   constructor •(dynamic value) → void
     ;
 }
 class DynamicReceiver1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method dynamicallyCalled(dynamic x) → dynamic
     ;
 }
 class DynamicReceiver2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     ;
   method dynamicallyCalled(dynamic x) → dynamic
     ;
 }
 static method staticMethod() → dynamic
   ;
 external static abstract method externalStatic() → core::bool;
 external static abstract method createBar() → self::Bar;
 static method stringArgument(dynamic x) → dynamic
   ;
 static method intArgument(dynamic x) → dynamic
   ;
 static method makeDynamicCall(dynamic receiver) → void
   ;
 static method main() → dynamic
   ;