Set 'isSyntheticDefault' for default constructors.

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

 library;
 import self as self;
 import "dart:core" as core;
 
 class A extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class B extends core::Object {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 class X extends core::Object implements self::A, self::B {
   field dynamic field = null;
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
 }
 static method useAsA(self::A object) → void {
   dynamic _ = object.{self::A::field};
 }
 static method useAsB(self::B object) → void {
   dynamic _ = object.{self::B::field};
   self::escape(object);
 }
 static method escape(dynamic x) → void {
   x.==(null) ? x = "" : null;
   x.==(null) ? x = 45 : null;
   if(!(x is core::int) && !(x is core::String)) {
     x.field = 45;
   }
 }
 static method main() → dynamic {
   self::X object = new self::X::•();
   self::useAsA(new self::A::•());
   self::useAsA(object);
   self::useAsB(new self::B::•());
   self::useAsB(object);
 }