fix ItemSeparatorComponent position in horizontal and inverted mode

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There's a positioning bug in `VirtualizedList` when `ItemSeparatorComponent` is defined for a list in horizontal or inverted mode. And also we face this bug in `FlatList`, because it is using `VirtualizedList` to render lists.
This commit will fix the [#15777](https://github.com/facebook/react-native/issues/15777).

Before fix:
```
<FlatList
  ...
  horizontal={true}
  inverted={true}
  ...
/>
```

![image](https://user-images.githubusercontent.com/15084663/30205251-95f14c70-949d-11e7-85e9-7a3304a52818.png)

```
<FlatList
  ...
  horizontal={true}
  inverted={false}
  ...
/>
```
![image](https://user-images.githubusercontent.com/15084663/30205411-f01d27b4-949d-11e7-991e-00aeae0c01e0.png)

I ran this code with all possible values of `horizontal` and `inverted` props in `FlatList` and `VirtualizedList` and the results of each run was as below:
After fix bug:

```
<FlatList
  ...
  horizontal={true}
  inverted={false}
  ...
/>
```
![image](https://user-images.githubusercontent.com/15084663/30205498-323bcf60-949e-11e7-8ba0-465614ea5cf2.png)

```
<FlatList
  ...
  horizontal={true}
  inverted={true}
  ...
/>
```
![image](https://user-images.githubusercontent.com/15084663/30205543-5274f612-949e-11e7-9660-bbdf8194cd27.png)
Closes https://github.com/facebook/react-native/pull/15865

Differential Revision: D5797266

Pulled By: hramos

fbshipit-source-id: 7d44fa797dbd9e83eb2bdd7833e9dd9707d9d822

Libraries/Lists/VirtualizedList.js

@@ -518,6 +518,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
       data,
       getItem,
       getItemCount,
+      horizontal,
       keyExtractor,
     } = this.props;
     const stickyOffset = this.props.ListHeaderComponent ? 1 : 0;
@@ -537,6 +538,7 @@ class VirtualizedList extends React.PureComponent<Props, State> {
           ItemSeparatorComponent={ii < end ? ItemSeparatorComponent : undefined}
           cellKey={key}
           fillRateHelper={this._fillRateHelper}
+          horizontal={horizontal}
           index={ii}
           inversionStyle={inversionStyle}
           item={item}
@@ -1255,6 +1257,7 @@ class CellRenderer extends React.Component<
     ItemSeparatorComponent: ?React.ComponentType<*>,
     cellKey: string,
     fillRateHelper: FillRateHelper,
+    horizontal: ?boolean,
     index: number,
     inversionStyle: ?StyleObj,
     item: Item,
@@ -1315,6 +1318,7 @@ class CellRenderer extends React.Component<
       CellRendererComponent,
       ItemSeparatorComponent,
       fillRateHelper,
+      horizontal,
       item,
       index,
       inversionStyle,
@@ -1336,9 +1340,14 @@ class CellRenderer extends React.Component<
     const itemSeparator =
       ItemSeparatorComponent &&
       <ItemSeparatorComponent {...this.state.separatorProps} />;
+    const cellStyle = inversionStyle
+      ? horizontal
+        ? [{flexDirection: 'row-reverse'}, inversionStyle]
+        : [{flexDirection: 'column-reverse'}, inversionStyle]
+      : horizontal ? [{flexDirection: 'row'}, inversionStyle] : inversionStyle;
     if (!CellRendererComponent) {
       return (
-        <View style={inversionStyle} onLayout={onLayout}>
+        <View style={cellStyle} onLayout={onLayout}>
           {element}
           {itemSeparator}
         </View>
@@ -1347,7 +1356,7 @@ class CellRenderer extends React.Component<
     return (
       <CellRendererComponent
         {...this.props}
-        style={inversionStyle}
+        style={cellStyle}
         onLayout={onLayout}>
         {element}
         {itemSeparator}

Libraries/Lists/__tests__/__snapshots__/VirtualizedList-test.js.snap

@@ -124,7 +124,14 @@ exports[`VirtualizedList handles nested lists 1`] = `
         <View>
           <View
             onLayout={[Function]}
-            style={null}
+            style={
+              Array [
+                Object {
+                  "flexDirection": "row",
+                },
+                null,
+              ]
+            }
           >
             <item
               title="outer1:inner0"
@@ -132,7 +139,14 @@ exports[`VirtualizedList handles nested lists 1`] = `
           </View>
           <View
             onLayout={[Function]}
-            style={null}
+            style={
+              Array [
+                Object {
+                  "flexDirection": "row",
+                },
+                null,
+              ]
+            }
           >
             <item
               title="outer1:inner1"
@@ -476,13 +490,18 @@ exports[`VirtualizedList renders all the bells and whistles 1`] = `
     <View
       onLayout={undefined}
       style={
+        Array [
+          Object {
+            "flexDirection": "column-reverse",
+          },
           Object {
             "transform": Array [
               Object {
                 "scaleY": -1,
               },
             ],
-        }
+          },
+        ]
       }
     >
       <item
@@ -493,13 +512,18 @@ exports[`VirtualizedList renders all the bells and whistles 1`] = `
     <View
       onLayout={undefined}
       style={
+        Array [
+          Object {
+            "flexDirection": "column-reverse",
+          },
           Object {
             "transform": Array [
               Object {
                 "scaleY": -1,
               },
             ],
-        }
+          },
+        ]
       }
     >
       <item
@@ -510,13 +534,18 @@ exports[`VirtualizedList renders all the bells and whistles 1`] = `
     <View
       onLayout={undefined}
       style={
+        Array [
+          Object {
+            "flexDirection": "column-reverse",
+          },
           Object {
             "transform": Array [
               Object {
                 "scaleY": -1,
               },
             ],
-        }
+          },
+        ]
       }
     >
       <item
@@ -527,13 +556,18 @@ exports[`VirtualizedList renders all the bells and whistles 1`] = `
     <View
       onLayout={undefined}
       style={
+        Array [
+          Object {
+            "flexDirection": "column-reverse",
+          },
           Object {
             "transform": Array [
               Object {
                 "scaleY": -1,
               },
             ],
-        }
+          },
+        ]
       }
     >
       <item
@@ -544,13 +578,18 @@ exports[`VirtualizedList renders all the bells and whistles 1`] = `
     <View
       onLayout={undefined}
       style={
+        Array [
+          Object {
+            "flexDirection": "column-reverse",
+          },
           Object {
             "transform": Array [
               Object {
                 "scaleY": -1,
               },
             ],
-        }
+          },
+        ]
       }
     >
       <item