Issue 5201 - [emscripten] Replace EM_ASM calls by a custom JavaScript library

compiled/String.h

Index: compiled/String.h
===================================================================
--- a/compiled/String.h
+++ b/compiled/String.h
@@ -21,16 +21,23 @@
 #include <cstddef>
 #include <cstring>
 #include <type_traits>
 
 #include <emscripten.h>
 
 #include "debug.h"
 
+extern "C"
+{
+  // It seems that calling JS is the easiest solution for lowercasing Unicode
+  // characters.
+  char16_t CharToLower(char16_t charCode);
+}
+
 inline void String_assert_readonly(bool readOnly);
 
 class String
 {
   friend class DependentString;
   friend class OwnedString;
 
 public:
@@ -183,21 +190,17 @@ public:
       value_type currChar = mBuf[i];
 
       // This should be more efficient with a lookup table but I couldn't measure
       // any performance difference.
       if (currChar >= u'A' && currChar <= u'Z')
         mBuf[i] = currChar + u'a' - u'A';
       else if (currChar >= 128)
       {
-        // It seems that calling JS is the easiest solution for lowercasing
-        // Unicode characters.
-        mBuf[i] = EM_ASM_INT({
-          return String.fromCharCode($0).toLowerCase().charCodeAt(0);
-        }, currChar);
+        mBuf[i] = CharToLower(currChar);
       }
     }
   }
 };
 
 class DependentString : public String
 {
 public: