Add "toString()" to BaseRequest.

pkg/http/test/request_test.dart

Index: pkg/http/test/request_test.dart
diff --git a/pkg/http/test/request_test.dart b/pkg/http/test/request_test.dart
index e26ee75e40ba4e44609eb6dcc500032fc9bca8dd..1b3d4cace964a16021ffb2e5a895c0ab1d3f8e25 100644
--- a/pkg/http/test/request_test.dart
+++ b/pkg/http/test/request_test.dart
@@ -387,5 +387,12 @@ void main() {
       expect(request.finalize, throwsStateError);
     });
   });
+
+  group('#toString()', () {
+    test('includes the method and URL', () {
+      var request = new http.Request('POST', dummyUrl);
+      expect(request.toString(), 'POST $dummyUrl');
+    });
+  });
 }