Update babel-preset, filter path by identifiers.

Summary:
This PR contains [the fix](#6063 (comment)) from #6063, which resolves the issue with rxjs 5. Is there a reason to not include these proposed changes in the repo?
Closes #10815

Differential Revision: D4226835

fbshipit-source-id: bfaeaebbe308c45bce814469ef0f75283f050d18

babel-preset/transforms/transform-symbol-member.js

@@ -24,12 +24,12 @@ module.exports = function symbolMember(babel) {
   return {
     visitor: {
       MemberExpression(path) {
-        let node = path.node;
-
-        if (!isAppropriateMember(node)) {
+        if (!isAppropriateMember(path)) {
           return;
         }
 
+        let node = path.node;
+
         path.replaceWith(
           t.conditionalExpression(
             t.binaryExpression(
@@ -54,8 +54,11 @@ module.exports = function symbolMember(babel) {
   };
 };
 
-function isAppropriateMember(node) {
-  return node.object.type === 'Identifier' &&
+function isAppropriateMember(path) {
+  let node = path.node;
+
+  return path.parentPath.type !== 'AssignmentExpression' &&
+    node.object.type === 'Identifier' &&
     node.object.name === 'Symbol' &&
     node.property.type === 'Identifier';
 }