Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 abstract class UsedAsBaseClass extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   abstract method usedInSubclass() → void;
   method calledFromB() → void {
     this.{self::UsedAsBaseClass::calledFromSubclass}();
   }
   abstract method calledFromSubclass() → void;
 }
 abstract class UsedAsInterface extends core::Object {
   abstract method usedInSubclass() → void;
 }
 class InstantiatedButMethodsUnused extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   method usedInSubclass() → void
     throw "Method removed by tree-shaking";
 }
 class ClassA extends self::UsedAsBaseClass implements self::UsedAsInterface, self::InstantiatedButMethodsUnused {
-  constructor •() → void
+  default constructor •() → void
     : super self::UsedAsBaseClass::•()
     ;
   method usedInSubclass() → void {
     core::print("A");
   }
 }
 class ClassB extends self::UsedAsBaseClass implements self::UsedAsInterface, self::InstantiatedButMethodsUnused {
-  constructor •() → void
+  default constructor •() → void
     : super self::UsedAsBaseClass::•()
     ;
   method usedInSubclass() → void {
     core::print("B");
     this.{self::UsedAsBaseClass::calledFromB}();
   }
   method calledFromSubclass() → void {}
 }
 static method baseClassCall(self::UsedAsBaseClass object) → void {
   object.{self::UsedAsBaseClass::usedInSubclass}();
@@ skipping 13 matching lines @@
   self::ClassA a = new self::ClassA::•();
   self::exactCallA(a);
   self::baseClassCall(a);
   self::interfaceCall(a);
   self::ClassB b = new self::ClassB::•();
   self::exactCallB(b);
   self::baseClassCall(b);
   self::interfaceCall(b);
   new self::InstantiatedButMethodsUnused::•();
 }