Set 'isSyntheticDefault' for default constructors.

pkg/front_end/testcases/implicit_scope_test.dart.direct.expect

 library;
 import self as self;
 import "dart:core" as core;
 import "package:expect/expect.dart" as exp;
 
 class ImplicitScopeTest extends core::Object {
-  constructor •() → void
+  default constructor •() → void
     : super core::Object::•()
     ;
   static method alwaysTrue() → core::bool {
     return 1.+(1).==(2);
   }
   static method testMain() → dynamic {
     dynamic a = "foo";
     dynamic b;
     if(self::ImplicitScopeTest::alwaysTrue()) {
       dynamic a = "bar";
@@ skipping 20 matching lines @@
       dynamic b = "baz";
     }
     while ("black".==("white"))
     exp::Expect::equals("foo", a);
     exp::Expect::equals(null, b);
   }
 }
 static method main() → dynamic {
   self::ImplicitScopeTest::testMain();
 }