Fix static warnings introduced by library cleanup.

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 1808277f633282423ef92aecb27748490edc3cd8..a99ca015a99f30f1d503617aca2e3748c120eaca 100644
--- a/sdk/lib/io/http_impl.dart
+++ b/sdk/lib/io/http_impl.dart
@@ -170,8 +170,11 @@ class _HttpClientResponse
   int get statusCode => _incoming.statusCode;
   String get reasonPhrase => _incoming.reasonPhrase;
 
-  X509Certificate get certificate =>
-    _httpRequest._httpClientConnection._socket.peerCertificate;
+  X509Certificate get certificate {
+    // The peerCertificate isn't on a plain socket, so cast to dynamic.
+    var socket = _httpRequest._httpClientConnection._socket;
+    return socket.peerCertificate;
+  }
 
   List<Cookie> get cookies {
     if (_cookies != null) return _cookies;