Support a user-supplied dart:io client in pkg/http.

pkg/http/lib/src/io_client.dart

Index: pkg/http/lib/src/io_client.dart
diff --git a/pkg/http/lib/src/io_client.dart b/pkg/http/lib/src/io_client.dart
index 9fd81acdfa865e1899dc26622edfdac8ca461989..34f622366c6d3fed76b4d5b3b4fdf699ecb5b965 100644
--- a/pkg/http/lib/src/io_client.dart
+++ b/pkg/http/lib/src/io_client.dart
@@ -20,9 +20,19 @@ class IOClient extends BaseClient {
   var _inner;
 
   /// Creates a new HTTP client.
-  IOClient() {
+  ///
+  /// [innerClient] must be a `dart:io` HTTP client. If it's not passed, a
+  /// default one will be instantiated.
+  IOClient([innerClient]) {
     io.assertSupported("IOClient");
-    _inner = io.newHttpClient();
+    if (innerClient != null) {
+      // TODO(nweiz): remove this assert when we can type [innerClient]
+      // properly.
+      assert(io.isHttpClient(innerClient));
+      _inner = innerClient;
+    } else {
+      _inner = io.newHttpClient();
+    }
   }
 
   /// Sends an HTTP request and asynchronously returns the response.