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

tests/standalone/io/http_client_exception_test.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.
 //
 
 import "dart:io";
 
 import "package:async_helper/async_helper.dart";
 import "package:expect/expect.dart";
 
 void testInvalidUrl() {
   HttpClient client = new HttpClient();
   Expect.throws(
-      () => client.getUrl(Uri.parse('ftp://www.google.com')));
+      () => client.getUrl(Uri.parse('ftp://www.google.com')),
+      (e) => e.toString().contains("Unsupported scheme"));
+  Expect.throws(
+      () => client.getUrl(Uri.parse('httpx://www.google.com')),
+      (e) => e.toString().contains("Unsupported scheme"));
+  Expect.throws(
+      () => client.getUrl(Uri.parse('http://::1')),
+      (e) => e.toString().contains("No host specified"));
+  Expect.throws(
+      () => client.getUrl(Uri.parse('http:///')),
+      (e) => e.toString().contains("No host specified"));
+  Expect.throws(
+      () => client.getUrl(Uri.parse('http:///index.html')),
+      (e) => e.toString().contains("No host specified"));
+  Expect.throws(
+      () => client.getUrl(Uri.parse('///')),
+      (e) => e.toString().contains("No host specified"));
+  Expect.throws(
+      () => client.getUrl(Uri.parse('///index.html')),
+      (e) => e.toString().contains("No host specified"));
 }
 
 void testBadHostName() {
   asyncStart();
   HttpClient client = new HttpClient();
   client.get("some.bad.host.name.7654321", 0, "/")
     .then((request) {
       Expect.fail("Should not open a request on bad hostname");
     }).catchError((error) {
       asyncEnd();  // We expect onError to be called, due to bad host name.
     }, test: (error) => error is! String);
 }
 
 void main() {
   testInvalidUrl();
   testBadHostName();
 }