Add a new component type, Profiler, that can be used to collect new render time metrics. Since this is a new, experimental API, it will be exported as React.unstable_Profiler initially.
Most of the functionality for this component has been added behind a feature flag, enableProfileModeMetrics. When the feature flag is disabled, the component will just render its children with no additional behavior. When the flag is enabled, React will also collect timing information and pass it to the onRender function (as described below).
* Add React.isValidElementType()
Per the conversation on #12453, there are a number of third-party
libraries (particularly those that generate higher-order components)
that are performing suboptimal validation of element types.
This commit exposes a function that can perform the desired check
without depending upon React internals.
* Move isValidElementType to shared/
Is this necessary? I'd like to use the package in enzyme to avoid having to recopy/paste the symbols for better debugging names, but at hard dep in enzyme proper on a version of react isn't gonna work. This seems safe since nothing explicitly depends on React in here?
* Support ForwardRef type of work in TestRenderer and ShallowRenderer.
* Release script now updates inter-package dependencies too (e.g. react-test-renderer depends on react-is).
