Close connections on server.close(), and call onError on early connection close.

tests/standalone/src/io/HttpServerEarlyCloseTest.dart

+#import("dart:io");
+#import("dart:isolate");
+
+class Server {
+  Server() {
+    HttpServer server = new HttpServer();
+    server.listen("127.0.0.1", 0);
+    port = server.port;
+    server.onRequest = (HttpRequest request, HttpResponse response) {
+      new Timer(100, (timer) => server.close());
+    };
+    server.onError = (Object exception) {
+      Expect.fail("Close should not give an error.");
+    };
+  }
+  int port;
+}
+
+class Client {
+  Client(int port) {
+    ReceivePort r = new ReceivePort();
+    HttpClient client = new HttpClient();
+    HttpClientConnection c = client.get("127.0.0.1", port, "/");
+    c.onRequest = (HttpClientRequest request) {
+      request.outputStream.close();
+    };
+    c.onResponse = (HttpClientResponse response) {
+      Expect.fail("Response should not be given, as not data was returned.");
+    };
+    c.onError = (Object exception) {
+      r.close();
+    };
+  }
+}
+
+main() {
+  Server server = new Server();
+  new Client(server.port);
+}