[react-events] Refactor getCurrentTarget to getResponderNode

[trueadm]

After discussion with @necolas, the behaviour of getCurrentTarget should be the same as the previous event.responderTarget – context.getResponderNode. This PR changes it so that this is the case.

In the case of scopes, they will always receive null for getResponderNode.

[sizebot]

Details of bundled changes.

Comparing: 9ff60ff...ad4115f

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-dom.profiling.min.js	0.0%	0.0%	115.32 KB	115.32 KB	36.35 KB	36.35 KB	NODE_PROFILING
react-dom-server.browser.development.js	0.0%	0.0%	142.1 KB	142.1 KB	37.12 KB	37.12 KB	UMD_DEV
ReactDOM-dev.js	-0.1%	-0.0%	939.62 KB	939.06 KB	207.48 KB	207.38 KB	FB_WWW_DEV
react-dom.development.js	-0.1%	-0.0%	915.48 KB	914.93 KB	207.45 KB	207.37 KB	UMD_DEV
react-dom.profiling.min.js	0.0%	-0.0%	115.12 KB	115.12 KB	37.08 KB	37.08 KB	UMD_PROFILING
react-dom.development.js	-0.1%	-0.0%	909.57 KB	909.02 KB	205.77 KB	205.69 KB	NODE_DEV
react-dom-server.node.development.js	0.0%	0.0%	139.05 KB	139.05 KB	36.34 KB	36.34 KB	NODE_DEV
react-dom.production.min.js	0.0%	-0.0%	111.68 KB	111.68 KB	35.39 KB	35.38 KB	NODE_PROD
react-dom-server.node.production.min.js	0.0%	0.0%	20.13 KB	20.13 KB	7.5 KB	7.51 KB	NODE_PROD
ReactDOM-prod.js	-0.2%	-0.1%	372.48 KB	371.89 KB	68.24 KB	68.15 KB	FB_WWW_PROD
ReactDOM-profiling.js	-0.2%	-0.1%	378.03 KB	377.44 KB	69.3 KB	69.21 KB	FB_WWW_PROFILING
react-dom-server.browser.production.min.js	0.0%	0.0%	19.73 KB	19.73 KB	7.34 KB	7.34 KB	NODE_PROD
react-dom-unstable-fizz.node.production.min.js	0.0%	🔺+0.2%	1.1 KB	1.1 KB	666 B	667 B	NODE_PROD

react-native-renderer

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
ReactFabric-prod.js	0.0%	-0.0%	261.69 KB	261.69 KB	44.95 KB	44.95 KB	RN_OSS_PROD
ReactFabric-dev.js	-0.1%	-0.1%	742.91 KB	742.37 KB	156.95 KB	156.86 KB	RN_FB_DEV
ReactFabric-prod.js	0.0%	-0.0%	261.7 KB	261.7 KB	44.96 KB	44.96 KB	RN_FB_PROD
ReactNativeRenderer-dev.js	0.0%	-0.0%	736.05 KB	736.05 KB	155.81 KB	155.81 KB	RN_FB_DEV
ReactNativeRenderer-profiling.js	0.0%	-0.0%	278.39 KB	278.39 KB	47.81 KB	47.81 KB	RN_FB_PROFILING
ReactFabric-dev.js	-0.1%	-0.1%	742.74 KB	742.2 KB	156.87 KB	156.78 KB	RN_OSS_DEV

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