Throw HttpException instead of String in http impl

This was noted in the flutter_tool crash reports, see flutter/flutter#38902, since we only catch specific exception types this ended up as an uncaught exception.

I think the HttpException class seems reasonably appropriate for this class of error.

Change-Id: I88801dfe558f5edb781513b0eb82f1544b980e89
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/113962
Reviewed-by: Zach Anderson <[email protected]>
Commit-Queue: Zach Anderson <[email protected]>

Author: jonahwilliams

sdk/lib/_http/http_impl.dart

@@ -1836,8 +1836,10 @@ class _HttpClientConnection {
     }
     return request.close().then((response) {
       if (response.statusCode != HttpStatus.ok) {
-        throw "Proxy failed to establish tunnel "
-            "(${response.statusCode} ${response.reasonPhrase})";
+        throw new HttpException(
+            "Proxy failed to establish tunnel "
+            "(${response.statusCode} ${response.reasonPhrase})",
+            uri: request.uri);
       }
       var socket = (response as _HttpClientResponse)
           ._httpRequest