Add ViewRegistry.prototype.getNextUpdatePromise

Signed-off-by: Max Brunsfeld <maxbrunsfeld@github.com>

src/view-registry.coffee

@@ -195,6 +195,10 @@ class ViewRegistry
   pollAfterNextUpdate: ->
     @performDocumentPollAfterUpdate = true
 
+  getNextUpdatePromise: ->
+    @nextUpdatePromise ?= new Promise (resolve) =>
+      @resolveNextUpdatePromise = resolve
+
   clearDocumentRequests: ->
     @documentReaders = []
     @documentWriters = []
@@ -220,6 +224,9 @@ class ViewRegistry
     # process updates requested as a result of reads
     writer() while writer = @documentWriters.shift()
 
+    @nextUpdatePromise = null
+    @resolveNextUpdatePromise?()
+
   startPollingDocument: ->
     window.addEventListener('resize', @requestDocumentPoll)
     @observer.observe(document, {subtree: true, childList: true, attributes: true})