Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class Foo extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method instanceMethod() → dynamic {
     return 123;
   }
 }
 abstract class ExternalValue extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 abstract class Bar extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   abstract method externalInstanceMethod() → self::ExternalValue;
 }
 class Box extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class FinalBox extends core::Object {
   final field dynamic finalField;
   constructor •(dynamic finalField) → void
     : self::FinalBox::finalField = finalField, super core::Object::•()
     ;
 }
 class SubFinalBox extends self::FinalBox {
   constructor •(dynamic value) → void
     : super self::FinalBox::•(value)
     ;
 }
 class DynamicReceiver1 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method dynamicallyCalled(dynamic x) → dynamic {}
 }
 class DynamicReceiver2 extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method dynamicallyCalled(dynamic x) → dynamic {}
 }
 static method staticMethod() → dynamic {
   return "sdfg";
 }
 external static abstract method externalStatic() → core::bool;
 external static abstract method createBar() → self::Bar;
 static method stringArgument(dynamic x) → dynamic {}
@@ skipping 13 matching lines @@
   dynamic a = box.{self::Box::field};
   self::FinalBox finalBox = new self::FinalBox::•("dfg");
   dynamic b = finalBox.{self::FinalBox::finalField};
   self::SubFinalBox subBox = new self::SubFinalBox::•("dfg");
   dynamic c = subBox.{self::FinalBox::finalField};
   self::makeDynamicCall(new self::DynamicReceiver1::•());
   self::makeDynamicCall(new self::DynamicReceiver2::•());
   core::List<core::String> list = <core::String>["string"];
   core::String d = list.{core::List::[]}(0);
 }