Provide better error message when the VM tries to load a script using HTTP

runtime/bin/builtin.dart

 // Copyright (c) 2012, 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.
 
 library builtin;
 import 'dart:io';
 // import 'root_library'; happens here from C Code
 
 // The root library (aka the script) is imported into this library. The
 // standalone embedder uses this to lookup the main entrypoint in the
@@ skipping 57 matching lines @@
   _httpRequestStatusString = error.toString();
   _httpRequestResponse = null;
 }
 
 
 void _makeHttpRequest(String uri) {
   var _client = new HttpClient();
   _httpRequestResponseCode = 0;
   _httpRequestStatusString = null;
   _httpRequestResponse = null;
-  Uri requestUri = Uri.parse(uri);
-  _client.getUrl(requestUri)
-      .then((HttpClientRequest request) => request.close())
-      .then((HttpClientResponse response) {
-        return response
-            .fold(new BytesBuilder(), (b, d) => b..add(d))
-            .then((builder) {
-              _requestCompleted(builder.takeBytes(), response);
-            });
-      }).catchError((error) {
-        _requestFailed(error);
-      });
+  try {
+    Uri requestUri = Uri.parse(uri);
+    _client.getUrl(requestUri)
+        .then((HttpClientRequest request) => request.close())
+        .then((HttpClientResponse response) {
+          return response
+              .fold(new BytesBuilder(), (b, d) => b..add(d))
+              .then((builder) {
+                _requestCompleted(builder.takeBytes(), response);
+              });
+        }).catchError((error) {
+          _requestFailed(error);
+        });
+  } catch (error) {
+    _requestFailed(error);
+  }
 }
 
 
 // Are we running on Windows?
 var _isWindows = false;
 var _workingWindowsDrivePrefix;
 // The current working directory
 var _workingDirectoryUri;
 // The URI that the entry point script was loaded from. Remembered so that
 // package imports can be resolved relative to it.
@@ skipping 158 matching lines @@
 
     throw "URIs using the 'package:' scheme should look like "
           "'$right', not '$wrong'.";
   }
 
   var packageRoot = _packageRoot == null ?
                     _entryPointScript.resolve('packages/') :
                     _packageRoot;
   return _filePathFromUri(packageRoot.resolve(uri.path).toString());
 }