Add responseType='arraybuffer' support to XHR

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="shell.sky" as="shell" />
+<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.responseText = "";
+    this.responseArrayBuffer = null;
+    this.responseText = null; // Cached to avoid re-decoding each access.
   }
 }
 
+// https://xhr.spec.whatwg.org
 class XMLHttpRequest {
   constructor() {
     this[kPrivate] = new Private;
+    this.responseType = ''; // Only text and arraybuffer support for now.
   }
 
   onload() {
   }
 
   onerror(error) {
   }
 
   get responseText() {
+    if (this.responseType !== '' && this.responseType !== 'text')
+      throw 'Non-text responseType ' + this.responseType;
+    if (this[kPrivate].responseArrayBuffer === null)
+      return null;
+    if (this[kPrivate].responseText === null) {
+      var intArray = new Uint8Array(this[kPrivate].responseArrayBuffer);
+      this[kPrivate].responseText = unicode.decodeUtf8String(intArray);
+    }
     return this[kPrivate].responseText;
   }
 
+  get response() {
+    if (this.responseType === 'text' || this.responseType == '')
+      return this.responseText;
+    else if (this.responseType === 'arraybuffer')
+      return this[kPrivate].responseArrayBuffer;
+    throw 'Unknown responseType ' + this.responseType;
+  }
+
   open(method, url) {
     var request = new loader.URLRequest();
     request.url = new URL(url, document.URL);
     request.method = method;
     request.auto_follow_redirects = true;
 
     var priv = this[kPrivate];
     priv.request = request;
     priv.headers.clear();
   }
@@ skipping 13 matching lines @@
     // FIXME: Factor this into the JS bindings.
     var pipe = new core.createMessagePipe();
     priv.networkService.createURLLoader(pipe.handle1);
     priv.loader = shell.wrapHandle(pipe.handle0, loader.URLLoader);
 
     var self = this;
     outstandingRequests.add(this);
     priv.loader.start(priv.request).then(function(result) {
       return core.drainData(result.response.body).then(function(result) {
         outstandingRequests.delete(self);
-        priv.responseText = unicode.decodeUtf8String(new Uint8Array(result.buffer));
+        priv.responseArrayBuffer = result.buffer;
+        // FIXME: Catch exceptions during onload so they don't trip onerror.
         self.onload();
       });
     }).catch(function(error) {
       outstandingRequests.delete(self);
       self.onerror(error);
     });
   }
 }
 
 module.exports = XMLHttpRequest;
 </script>