Injected Host Component classes are not being considered by the reconciler (#8050)

* Consider Host Component classes when creating a new internal instance

* Remove unused tagToComponentClass & injectComponentClasses from ReactHostComponent

(cherry picked from commit 461a74115c)

src/renderers/shared/stack/reconciler/ReactHostComponent.js

@@ -14,8 +14,6 @@
 var invariant = require('invariant');
 
 var genericComponentClass = null;
-// This registry keeps track of wrapper classes around host tags.
-var tagToComponentClass = {};
 var textComponentClass = null;
 
 var ReactHostComponentInjection = {
@@ -29,11 +27,6 @@ var ReactHostComponentInjection = {
   injectTextComponentClass: function(componentClass) {
     textComponentClass = componentClass;
   },
-  // This accepts a keyed object with classes as values. Each key represents a
-  // tag. That particular tag will use this class instead of the generic one.
-  injectComponentClasses: function(componentClasses) {
-    Object.assign(tagToComponentClass, componentClasses);
-  },
 };
 
 /**