Fix ReactDOMSelection for IE 11

IE 11 no longer supports the legacy document.selection API.
Their implementation of window.getSelection() doesn't support
the extend() method, which we were relying on.

If the selection is RTL and selection extend is missing, then just
flip the selection.

src/core/ReactDOMSelection.js

@@ -137,7 +137,15 @@ function setModernOffsets(node, offsets) {
   var length = node[getTextContentAccessor()].length;
   var start = Math.min(offsets.start, length);
   var end = typeof offsets.end === 'undefined' ?
-    start : Math.min(offsets.end, length);
+            start : Math.min(offsets.end, length);
+
+  // IE 11 uses modern selection, but doesn't support the extend method.
+  // Flip backward selections, so we can set with a single range.
+  if (!selection.extend && start > end) {
+    var temp = end;
+    end = start;
+    start = temp;
+  }
 
   var startMarker = getNodeForCharacterOffset(node, start);
   var endMarker = getNodeForCharacterOffset(node, end);
@@ -146,8 +154,15 @@ function setModernOffsets(node, offsets) {
     var range = document.createRange();
     range.setStart(startMarker.node, startMarker.offset);
     selection.removeAllRanges();
-    selection.addRange(range);
-    selection.extend(endMarker.node, endMarker.offset);
+
+    if (start > end) {
+      selection.addRange(range);
+      selection.extend(endMarker.node, endMarker.offset);
+    } else {
+      range.setEnd(endMarker.node, endMarker.offset);
+      selection.addRange(range);
+    }
+
     range.detach();
   }
 }