Make anchoring implementation constructor-injectable in Guest class

This enables mocking of the `anchoring/html` module in tests without having
to patch the `anchoring` property after the class is constructed.

src/annotator/guest.coffee

@@ -38,7 +38,7 @@ module.exports = class Guest extends Delegator
     TextSelection: {}
 
   # Anchoring module
-  anchoring: htmlAnchoring
+  anchoring: null
 
   # Internal state
   plugins: null
@@ -49,7 +49,7 @@ module.exports = class Guest extends Delegator
   html:
     adder: '<hypothesis-adder></hypothesis-adder>'
 
-  constructor: (element, config) ->
+  constructor: (element, config, anchoring = htmlAnchoring) ->
     super
 
     this.adder = $(this.html.adder).appendTo(@element).hide()
@@ -78,6 +78,8 @@ module.exports = class Guest extends Delegator
     # The "top" guest instance will have this as null since it's in a top frame not a sub frame
     this.frameIdentifier = config.subFrameIdentifier || null
 
+    this.anchoring = anchoring
+
     cfOptions =
       config: config
       on: (event, handler) =>

src/annotator/test/guest-test.coffee

@@ -54,7 +54,6 @@ describe 'Guest', ->
     config = Object.assign({}, guestConfig, config)
     element = document.createElement('div')
     guest = new Guest(element, config)
-    guest.anchoring = htmlAnchoring
     return guest
 
   beforeEach ->
@@ -77,6 +76,7 @@ describe 'Guest', ->
 
     Guest.$imports.$mock({
       './adder': {Adder: FakeAdder},
+      './anchoring/html': htmlAnchoring,
       './highlighter': highlighter,
       './range-util': rangeUtil,
       './selections': (document) ->