Dash

Fixed
- [999](https://github.com/plotly/dash/pull/999) Fix fingerprint for component suites with `metadata` in version.
- [983](https://github.com/plotly/dash/pull/983) Fix the assets loading issues when dashR application runner is handling with an app defined by string chunk.

Fixed
- [987](https://github.com/plotly/dash/pull/987) Fix cache string handling for component suites with nested folders in their packages.
- [986](https://github.com/plotly/dash/pull/986) Fix a bug with evaluation of `_force_eager_loading` when application is loaded with gunicorn

Added
- [964](https://github.com/plotly/dash/pull/964) Adds support for preventing updates in clientside functions.
- Reject all updates with `throw window.dash_clientside.PreventUpdate;`
- Reject a single output by returning `window.dash_clientside.no_update`
- [899](https://github.com/plotly/dash/pull/899) Add support for async dependencies and components
- [973](https://github.com/plotly/dash/pull/973) Adds support for resource caching and adds a fallback caching mechanism through etag

Fixed
- [974](https://github.com/plotly/dash/pull/974) Fix and improve a percy snapshot behavior issue we found in dash-docs testing. It adds a flag `wait_for_callbacks` to ensure that, in the context of a dash app testing, the percy snapshot action will happen only after all callbacks get fired.

Fixed
- [969](https://github.com/plotly/dash/pull/969) Fix warnings emitted by react devtools coming from our own devtools components.

Added
- [948](https://github.com/plotly/dash/pull/948) Support setting working directory for R apps run using the `dashr` fixture, primarily useful for tests with assets. `dashr.start_server` supports a `cwd` argument to set an explicit working directory, and has smarter defaults when it's omitted: if `app` is a path to an R script, uses the directory of that path; if `app` is a string, uses the directory the test file itself is in.
- [944](https://github.com/plotly/dash/pull/944)
- Relevant `dash.testing` methods can now be called with either an element or a CSS selector: `select_dcc_dropdown`, `multiple_click`, `clear_input`, `zoom_in_graph_by_ratio`, `click_at_coord_fractions`.
- Three new `dash.testing` methods: `clear_local_storage`, `clear_session_storage`, and `clear_storage` (to clear both together)
- [937](https://github.com/plotly/dash/pull/937) `dash.testing` adds two APIs `zoom_in_graph_by_ratio` and `click_at_coord_fractions` about advanced interactions using mouse `ActionChain`
- [938](https://github.com/plotly/dash/issues/938) Add debugging traces to dash backend about serving component suites, to verify the installed packages whenever in doubt.

Fixed
- [944](https://github.com/plotly/dash/pull/944) Fix a bug with persistence being toggled on/off on an existing component.

Changed
- Bump dash-core-components version from 1.2.0 to [1.2.1](https://github.com/plotly/dash-core-components/blob/master/CHANGELOG.md#120---2019-09-19)