Issue 4125 - [emscripten] Convert filter classes to C++

compiled/StringScanner.h

 #pragma once
 
 #include "String.h"
 
 class StringScanner
 {
 private:
   const DependentString mStr;
   String::size_type mPos;
   String::size_type mEnd;
   String::value_type mTerminator;
 public:
-  StringScanner(const String& str, String::size_type pos = 0,
+  explicit StringScanner(const String& str, String::size_type pos = 0,
         String::value_type terminator = u'\0')
       : mStr(str), mPos(pos), mEnd(str.length()), mTerminator(terminator) {}
 
-  bool done()
+  bool done() const
   {
     return mPos >= mEnd;
   }
 
-  String::size_type position()
+  String::size_type position() const
   {
     return mPos - 1;
   }
 
   String::value_type next()
   {
     String::value_type result = done() ? mTerminator : mStr[mPos];
     mPos++;
     return result;
   }
 
   bool skipOne(String::value_type ch)
   {
     if (!done() && mStr[mPos] == ch)
     {
       mPos++;
       return true;
     }
 
     return false;
   }
 
   bool skip(String::value_type ch)
   {
-    String::size_type oldPos = mPos;
-    while (!done() && mStr[mPos] == ch)
-      mPos++;
-    return oldPos != mPos;
+    bool skipped = false;
+    while ((skipped = skipOne(ch)));
+    return skipped;
   }
 };