Improve HTTP error message

sdk/lib/io/http_impl.dart

Index: sdk/lib/io/http_impl.dart
diff --git a/sdk/lib/io/http_impl.dart b/sdk/lib/io/http_impl.dart
index 7b80d6a7c14802161fa92ea5e68f7f3087e33fe2..971186d9f3ee1edb3d1f2186caba2a124aeb0557 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -1280,7 +1280,9 @@ class _HttpClientConnection {
           _subscription.pause();
           // We assume the response is not here, until we have send the request.
           if (_nextResponseCompleter == null) {
-            throw new HttpException("Unexpected response.", uri: _currentUri);
+            throw new HttpException(
+                "Unexpected response (unsolicited response without request).",
+                uri: _currentUri);
           }
           _nextResponseCompleter.complete(incoming);
           _nextResponseCompleter = null;