Fix anchoring using quote selector alone failing

Looking at the `anchor` function, it is clearly intended that the
position selector is optional for anchoring, as is the case for HTML
anchoring. However, anchoring using only a quote selector did not
actually work. This was in part due to the bug fixed in the previous
commit but also because of a missing check for a null position selector
when populating the (quote,position) cache.

In order for the test to pass using only a quote, it was necessary to
change the search string to a unique quote which only occurs once in the
document. Otherwise the quote anchoring found the occurrence of
'Netherfield Park' on the non-rendered page 1.

src/annotator/anchoring/pdf.coffee

@@ -8,8 +8,11 @@ RenderingStates = require('../pdfjs-rendering-states')
 
 # Caches for performance
 
-# Map of page index to Promise<string>
+# Map of page index to page text content as a `Promise<string>`
 pageTextCache = {}
+# Two-dimensional map from `[quote][position]` to `{page, anchor}` intended to
+# optimize re-anchoring of a pair of quote and position selectors if the
+# position selector fails to anchor on its own.
 quotePositionCache = {}
 
 
@@ -132,7 +135,7 @@ anchorByPosition = (page, anchor, options) ->
 # Returns a `Promise<Range>` for the location of the quote.
 findInPages = ([pageIndex, rest...], quote, position) ->
   unless pageIndex?
-    return Promise.reject('quote not found')
+    return Promise.reject(new Error('Quote not found'))
 
   attempt = (info) ->
     # Try to find the quote in the current page.
@@ -151,8 +154,9 @@ findInPages = ([pageIndex, rest...], quote, position) ->
     return findInPages(rest, quote, position)
 
   cacheAndFinish = (anchor) ->
-    quotePositionCache[quote.exact] ?= {}
-    quotePositionCache[quote.exact][position.start] = {page, anchor}
+    if position
+      quotePositionCache[quote.exact] ?= {}
+      quotePositionCache[quote.exact][position.start] = {page, anchor}
     return anchorByPosition(page, anchor)
 
   page = getPage(pageIndex)

src/annotator/anchoring/test/pdf-test.js

@@ -139,7 +139,7 @@ describe('PDF anchoring', function () {
   describe('#anchor', function () {
     it('anchors previously created selectors if the page is rendered', function () {
       viewer.setCurrentPage(2);
-      var range = findText(container, 'Netherfield Park');
+      var range = findText(container, 'My dear Mr. Bennet');
       return pdfAnchoring.describe(container, range).then(function (selectors) {
         var position = selectors[0];
         var quote = selectors[1];
@@ -149,8 +149,7 @@ describe('PDF anchoring', function () {
         var subsets = [
           [position, quote],
           [position],
-          // FIXME - Anchoring a quote on its own does not currently work
-          // [quote],
+          [quote],
         ];
         var subsetsAnchored = subsets.map(function (subset) {
           var types = subset.map(function (s) { return s.type; });