Removed optimization for events without target in ReactNativeEventEmi…

…tter

This PR fixes the problem originally introduced in facebook#6590
The problem is that `ResponderEventPlugin` (and `ResponderTouchHistoryStore`) relies on that fact that touch events are balanced.
So if one `startish` event happened, should be coming `endish` event. Otherwise there is no way to maintain internal `trackedTouchCount` counter. So, if we drop some events, we break this logic.

Moreover, that optimization clearly contradict with this statement from `ResponderEventPlugin`:
```
We must be resilient to `targetInst` being `null` on `touchMove` or
`touchEnd`. On certain platforms, this means that a native scroll has
assumed control and the original touch targets are destroyed.
```

This issue causes several major problems in React Native, and one of them (finally!) is easy to reproduce: facebook/react-native#12976 .

The test also illustrates this problem.

scripts/fiber/tests-passing.txt

@@ -1438,6 +1438,7 @@ src/renderers/native/__tests__/ReactNativeEvents-test.js
 * handles events
 * handles events on text nodes
 * handles when a responder is unmounted while a touch sequence is in progress
+* handles events without target
 
 src/renderers/native/__tests__/ReactNativeMount-test.js
 * should be able to create and render a native component

src/renderers/native/ReactNativeEventEmitter.js

@@ -102,11 +102,6 @@ var ReactNativeEventEmitter = {
   ) {
     var nativeEvent = nativeEventParam || EMPTY_NATIVE_EVENT;
     var inst = ReactNativeComponentTree.getInstanceFromNode(rootNodeID);
-    if (!inst) {
-      // If the original instance is already gone, we don't have to dispatch
-      // any events.
-      return;
-    }
     ReactGenericBatching.batchedUpdates(function() {
       ReactNativeEventEmitter.handleTopLevel(
         topLevelType,

src/renderers/native/__tests__/ReactNativeEvents-test.js

@@ -267,3 +267,88 @@ it('handles when a responder is unmounted while a touch sequence is in progress'
   expect(getResponderId()).toBe('two');
   expect(log).toEqual(['two responder start']);
 });
+
+it('handles events without target', () => {
+  var EventEmitter = RCTEventEmitter.register.mock.calls[0][0];
+  var View = createReactNativeComponentClass({
+    validAttributes: {id: true},
+    uiViewClassName: 'View',
+  });
+
+  function getViewById(id) {
+    return UIManager.createView.mock.calls.find(
+      args => args[3] && args[3].id === id,
+    )[0];
+  }
+
+  function getResponderId() {
+    const responder = ResponderEventPlugin._getResponder();
+    if (responder === null) {
+      return null;
+    }
+    const props = typeof responder.tag === 'number'
+      ? responder.memoizedProps
+      : responder._currentElement.props;
+    return props ? props.id : null;
+  }
+
+  var log = [];
+  ReactNative.render(
+    <View id="parent">
+      <View key={1}>
+        <View
+          id="one"
+          onResponderEnd={() => log.push('one responder end')}
+          onResponderStart={() => log.push('one responder start')}
+          onStartShouldSetResponder={() => true}
+        />
+      </View>
+      <View key={2}>
+        <View
+          id="two"
+          onResponderEnd={() => log.push('two responder end')}
+          onResponderStart={() => log.push('two responder start')}
+          onStartShouldSetResponder={() => true}
+        />
+      </View>
+    </View>,
+    1,
+  );
+
+  EventEmitter.receiveTouches(
+    'topTouchStart',
+    [{target: getViewById('one'), identifier: 17}],
+    [0],
+  );
+
+  EventEmitter.receiveTouches(
+    'topTouchEnd',
+    [{target: null, identifier: 17}],
+    [0],
+  );
+
+  expect(getResponderId()).toBe(null);
+
+  EventEmitter.receiveTouches(
+    'topTouchStart',
+    [{target: getViewById('two'), identifier: 18}],
+    [0],
+  );
+
+  expect(getResponderId()).toBe('two');
+
+  EventEmitter.receiveTouches(
+    'topTouchEnd',
+    [{target: getViewById('two'), identifier: 18}],
+    [0],
+  );
+
+  expect(getResponderId()).toBe(null);
+
+  expect(log).toEqual([
+    'one responder start',
+    'one responder end',
+    'two responder start',
+    'two responder end',
+  ]);
+});