Use Map instead of object as map in ReactNativeComponentTree (facebook#16107)

Real Maps should now be used in RN JS engines. In theory this should
be faster (but might not actually be in practice), and it avoids hitting
upper bounds of property max counts.

We don't use these types of Maps in Fabric.

Author: sebmarkbage

packages/react-native-renderer/src/ReactNativeComponentTree.js

@@ -7,20 +7,20 @@
 
 import invariant from 'shared/invariant';
 
-const instanceCache = {};
-const instanceProps = {};
+const instanceCache = new Map();
+const instanceProps = new Map();
 
 export function precacheFiberNode(hostInst, tag) {
-  instanceCache[tag] = hostInst;
+  instanceCache.set(tag, hostInst);
 }
 
 export function uncacheFiberNode(tag) {
-  delete instanceCache[tag];
-  delete instanceProps[tag];
+  instanceCache.delete(tag);
+  instanceProps.delete(tag);
 }
 
 function getInstanceFromTag(tag) {
-  return instanceCache[tag] || null;
+  return instanceCache.get(tag) || null;
 }
 
 function getTagFromInstance(inst) {
@@ -39,9 +39,9 @@ export {
 };
 
 export function getFiberCurrentPropsFromNode(stateNode) {
-  return instanceProps[stateNode._nativeTag] || null;
+  return instanceProps.get(stateNode._nativeTag) || null;
 }
 
 export function updateFiberProps(tag, props) {
-  instanceProps[tag] = props;
+  instanceProps.set(tag, props);
 }