[js] cannot create property 'hx__closures__' on string

hxcpp-debugger currently doesn't compile (the last CI build ran in January):

``` haxe
import debugger.DebuggerThread;

class Main {
public static function main() {}
}
```

```
./src/debugger/_DebuggerThread/TypeHelpers.cpp(466): error C2819: type 'String' does not have an overloaded member 'operator ->'
c:\haxetoolkit\haxe\lib\hxcpp\git\include\hxString.h(20): note: see declaration of 'String'
./src/debugger/_DebuggerThread/TypeHelpers.cpp(466): note: did you intend to use '.' instead?
./src/debugger/_DebuggerThread/TypeHelpers.cpp(466): error C2232: '->String::toLowerCase_dyn': left operand has 'class' type, use '.'
./src/debugger/_DebuggerThread/TypeHelpers.cpp(466): error C2660: 'Reflect_obj::compare': function does not take 1 arguments
```

I wanted to use it to debug some hxcpp regressions. There's a joke in there somewhere...

Reproduce the failure with the Haxe Main example, then inspect src/debugger/_DebuggerThread/TypeHelpers.cpp at line 466 alongside hxString.h. Trace the generated String and Reflect_obj::compare usage; done means hxcpp-debugger compiles without the reported C2819, C2232, and C2660 errors.

Written by the indexing model from the issue text.