Fix the scoping of the initial value for the `plugins` property

This fixes the remaining failing tests.
When compiled from coffeescript the initial `{}` value was at the prototype level instead of scoped at the function level.
It looked like I was declaring it as an instance variable, but it was more like a class variable.. coffeescript is weird to me..

src/annotator/guest.coffee

@@ -48,6 +48,7 @@ module.exports = class Guest extends Delegator
   anchoring: require('./anchoring/html')
 
   # Internal state
+  plugins: null
   anchors: null
   visibleHighlights: false
 
@@ -55,8 +56,6 @@ module.exports = class Guest extends Delegator
     adder: '<hypothesis-adder></hypothesis-adder>'
     wrapper: '<div class="annotator-wrapper"></div>'
 
-  plugins: {}
-
   addPlugin: (name, options) ->
     if @plugins[name]
       console.error("You cannot have more than one instance of any plugin.")
@@ -92,6 +91,7 @@ module.exports = class Guest extends Delegator
         else
           self._onClearSelection()
 
+    this.plugins = {}
     this.anchors = []
 
     cfOptions =