Use existing unicode.sky instead of re-inventing

sky/framework/xmlhttprequest.sky

 <import src="/mojo/public/sky/core.sky" as="core" />
 <import src="/mojo/public/sky/unicode.sky" as="unicode" />
 <import src="/mojo/services/network/public/interfaces/network_service.mojom.sky" as="net" />
 <import src="/mojo/services/network/public/interfaces/url_loader.mojom.sky" as="loader" />
 <import src="/sky/framework/shell.sky" as="shell" />
 <script>
 // XHR keeps itself alive.
 var outstandingRequests = new Set();
 
 const kPrivate = Symbol("XMLHttpRequestPrivate");
 
 class Private {
   constructor() {
     this.networkService = shell.connectToService(
       "mojo:network_service", net.NetworkService);
     this.request = null;
     this.loader = null;
     this.headers = new Map();
     this.responseArrayBuffer = null;
     this.responseText = null; // Cached to avoid re-decoding each access.
   }
 }
 
-// Somewhat hacky, but works.
-function stringToUTF8Buffer(string) {
-    var string = unescape(encodeURIComponent(string));
-    var charList = string.split('');
-    var uintArray = [];
-    for (var i = 0; i < charList.length; i++) {
-        uintArray.push(charList[i].charCodeAt(0));
-    }
-    return new Uint8Array(uintArray);
-}
-
 // https://xhr.spec.whatwg.org
 class XMLHttpRequest {
   constructor() {
     this[kPrivate] = new Private;
     this.responseType = ''; // Only text and arraybuffer support for now.
     this.status = null;
     this.statusText = null;
   }
 
   onload() {
@@ skipping 37 matching lines @@
     this[kPrivate].headers.set(header, value);
   }
 
   send(body) {
     var priv = this[kPrivate];
     // Handle the body before the headers as it can affect Content-Type.
     if (body) {
       var bodyAsBufferView = null;
       if (typeof(body) === "string") {
         this.setRequestHeader("Content-Type", "text/plain;charset=UTF-8");
-        bodyAsBufferView = stringToUTF8Buffer(body);
+        var bodyAsBufferView = new Uint8Array(unicode.utf8Length(body));
+        unicode.encodeUtf8String(body, bodyAsBufferView);
       } else {
         bodyAsBufferView = new Uint8Array(body);
       }
       var dataPipe = new core.createDataPipe();
       // FIXME: body is currently assumed to be an ArrayBuffer.
       var writeResult = core.writeData(dataPipe.producerHandle,
         bodyAsBufferView, core.WRITE_DATA_FLAG_ALL_OR_NONE);
       core.close(dataPipe.producerHandle);
       // FIXME: Much better error handling needed.
       console.assert(writeResult.result === core.RESULT_OK);
@@ skipping 31 matching lines @@
     }).catch(function(error) {
       outstandingRequests.delete(self);
       // Technically this should throw a ProgressEvent.
       self.onerror(error);
     });
   }
 }
 
 module.exports = XMLHttpRequest;
 </script>