[Flare] Switch from currentTarget model to responderTarget model

[trueadm]

This PR changes responder event.currentTarget to event.responderTarget. This also works a bit differently, in that it will give you the last currentTarget for the specific responder you're working with. This ensures that the currentTarget remains consistent between onEvent and onRootEvent as well usage via hooks.

[sizebot]

Details of bundled changes.

Comparing: 67e3f3f...f5e6c6e

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
ReactDOM-dev.js	0.0%	0.0%	914.19 KB	914.3 KB	203.19 KB	203.2 KB	FB_WWW_DEV
react-dom-test-utils.development.js	0.0%	0.0%	57.96 KB	57.96 KB	15.91 KB	15.91 KB	UMD_DEV
react-dom-unstable-fizz.browser.development.js	0.0%	+0.1%	3.78 KB	3.78 KB	1.52 KB	1.52 KB	UMD_DEV
react-dom-test-utils.production.min.js	0.0%	-0.0%	10.95 KB	10.95 KB	4.01 KB	4.01 KB	UMD_PROD
react-dom-test-utils.production.min.js	0.0%	-0.1%	10.71 KB	10.71 KB	3.95 KB	3.94 KB	NODE_PROD
react-dom.development.js	0.0%	0.0%	891.15 KB	891.54 KB	202.82 KB	202.84 KB	UMD_DEV
react-dom.development.js	0.0%	0.0%	885.44 KB	885.83 KB	201.32 KB	201.34 KB	NODE_DEV
ReactDOM-prod.js	🔺+0.2%	🔺+0.1%	370.39 KB	371.12 KB	68.02 KB	68.05 KB	FB_WWW_PROD
ReactDOM-profiling.js	+0.2%	0.0%	376.72 KB	377.44 KB	69.38 KB	69.4 KB	FB_WWW_PROFILING
react-dom-server.browser.development.js	0.0%	0.0%	131.88 KB	131.88 KB	34.88 KB	34.89 KB	NODE_DEV
react-dom-unstable-native-dependencies.development.js	0.0%	-0.0%	60.76 KB	60.76 KB	15.85 KB	15.85 KB	UMD_DEV
react-dom-unstable-native-dependencies.production.min.js	0.0%	0.0%	10.74 KB	10.74 KB	3.68 KB	3.68 KB	UMD_PROD

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