Throw a useful error when injecting tap event plugin more than once

This will prevent people from receiving a cryptic error message
from React about two plugins with the same name being injected.

injectTapEventPlugin() should only be called once per application,
and ideally at the entry point.

src/injectTapEventPlugin.js

@@ -1,10 +1,26 @@
-var defaultClickRejectionStrategy = require("./defaultClickRejectionStrategy");
+var invariant = require('fbjs/lib/invariant');
+var defaultClickRejectionStrategy = require('./defaultClickRejectionStrategy');
+
+var alreadyInjected = false;
 
 module.exports = function injectTapEventPlugin (strategyOverrides) {
   strategyOverrides = strategyOverrides || {}
   var shouldRejectClick = strategyOverrides.shouldRejectClick || defaultClickRejectionStrategy;
 
+  if (process.env.NODE_ENV !== 'production') {
+    invariant(
+      !alreadyInjected,
+      'injectTapEventPlugin(): Can only be called once per application lifecycle.\n\n\
+It is recommended to call injectTapEventPlugin() just before you call \
+ReactDOM.render(). If you are using an external library which calls injectTapEventPlugin() \
+itself, please contact the maintainer as it shouldn\'t be called in library code and \
+should be injected by the application.'
+    )
+  }
+
+  alreadyInjected = true;
+
   require('react/lib/EventPluginHub').injection.injectEventPluginsByName({
-    "TapEventPlugin":       require('./TapEventPlugin.js')(shouldRejectClick)
+    'TapEventPlugin':       require('./TapEventPlugin.js')(shouldRejectClick)
   });
 };