Improve the HTTP client error when the provided URI is not suitable

sdk/lib/io/http_impl.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of dart.io;
 
 class _HttpIncoming extends Stream<List<int>> {
   final int _transferLength;
   final Completer _dataCompleter = new Completer();
   Stream<List<int>> _stream;
@@ skipping 1560 matching lines @@
     _proxyCredentials.add(new _ProxyCredentials(host, port, realm, cr));
   }
 
   set findProxy(String f(Uri uri)) => _findProxy = f;
 
   Future<HttpClientRequest> _openUrl(String method, Uri uri) {
     if (method == null) {
       throw new ArgumentError(method);
     }
     if (method != "CONNECT") {
-      if (uri.host.isEmpty ||
-          (uri.scheme != "http" && uri.scheme != "https")) {
-        throw new ArgumentError("Unsupported scheme '${uri.scheme}' in $uri");
+      if (uri.host.isEmpty) {
+        throw new ArgumentError("No host specified in URI $uri");
+      } else if (uri.scheme != "http" && uri.scheme != "https") {
+        throw new ArgumentError(
+            "Unsupported scheme '${uri.scheme}' in URI $uri");
       }
     }
 
     bool isSecure = (uri.scheme == "https");
     int port = uri.port;
     if (port == 0) {
       port = isSecure ?
           HttpClient.DEFAULT_HTTPS_PORT :
           HttpClient.DEFAULT_HTTP_PORT;
     }
@@ skipping 925 matching lines @@
   final Uri location;
 }
 
 String _getHttpVersion() {
   var version = Platform.version;
   // Only include major and minor version numbers.
   int index = version.indexOf('.', version.indexOf('.') + 1);
   version = version.substring(0, index);
   return 'Dart/$version (dart:io)';
 }