Extract `normalizeURI` function into a utility module

Extract the existing `normalizeURI` function from guest.coffee into a
utility module and re-use it in document.coffee to convert relative URLs
to absolute ones.

Also implement some direct tests for this function.

src/annotator/guest.coffee

-baseURI = require('document-base-uri')
 extend = require('extend')
 raf = require('raf')
 scrollIntoView = require('scroll-into-view')
@@ -12,6 +11,7 @@ highlighter = require('./highlighter')
 rangeUtil = require('./range-util')
 selections = require('./selections')
 xpathRange = require('./anchoring/range')
+{ normalizeURI } = require('./util/url')
 
 animationPromise = (fn) ->
   return new Promise (resolve, reject) ->
@@ -21,17 +21,6 @@ animationPromise = (fn) ->
       catch error
         reject(error)
 
-# Normalize the URI for an annotation. This makes it absolute and strips
-# the fragment identifier.
-normalizeURI = (uri, baseURI) ->
-  # Convert to absolute URL
-  url = new URL(uri, baseURI)
-  # Remove the fragment identifier.
-  # This is done on the serialized URL rather than modifying `url.hash` due
-  # to a bug in Safari.
-  # See https://github.com/hypothesis/h/issues/3471#issuecomment-226713750
-  return url.toString().replace(/#.*/, '');
-
 module.exports = class Guest extends Delegator
   SHOW_HIGHLIGHTS_CLASS = 'annotator-highlights-always-on'
 
@@ -140,7 +129,7 @@ module.exports = class Guest extends Delegator
 
     return Promise.all([metadataPromise, uriPromise]).then ([metadata, href]) =>
       return {
-        uri: normalizeURI(href, baseURI),
+        uri: normalizeURI(href),
         metadata,
         frameIdentifier: this.frameIdentifier
       }

src/annotator/plugin/document.coffee

 $ = require('jquery')
-Plugin = require('../plugin')
-
 baseURI = require('document-base-uri')
 
+Plugin = require('../plugin')
+{ normalizeURI } = require('../util/url')
+
 ###
 ** Adapted from:
 ** https://github.com/openannotation/annotator/blob/v1.2.x/src/plugin/document.coffee
@@ -185,9 +186,7 @@ module.exports = class Document extends Plugin
 
   # Hack to get a absolute url from a possibly relative one
   _absoluteUrl: (url) ->
-    d = @document.createElement('a')
-    d.href = url
-    d.href
+    normalizeURI(url, @baseURI)
 
   # Get the true URI record when it's masked via a different protocol.
   # This happens when an href is set with a uri using the 'blob:' protocol

src/annotator/util/test/url-test.js

+'use strict';
+
+const { normalizeURI } = require('../url');
+
+describe('annotator.util.url', () => {
+  describe('normalizeURI', () => {
+    it('resolves relative URLs against the provided base URI', () => {
+      const base = 'http://example.com';
+      assert.equal(normalizeURI('index.html', base), `${base}/index.html`);
+    });
+
+    it('resolves relative URLs against the document URI, if no base URI is provided', () => {
+      // Strip filename from base URI.
+      const base = document.baseURI.replace(/\/[^/]*$/, '');
+      assert.equal(normalizeURI('foo.html'), `${base}/foo.html`);
+    });
+
+    it('does not modify absolute URIs', () => {
+      const url = 'http://example.com/wibble';
+      assert.equal(normalizeURI(url), url);
+    });
+
+    it('removes the fragment identifier', () => {
+      const url = 'http://example.com/wibble#fragment';
+      assert.equal(normalizeURI(url), 'http://example.com/wibble');
+    });
+
+    [
+      'file:///Users/jane/article.pdf',
+      'doi:10.1234/4567',
+    ].forEach(url => {
+      it('does not modify absolute non-HTTP/HTTPS URLs', () => {
+        assert.equal(normalizeURI(url), url);
+      });
+    });
+  });
+});

src/annotator/util/url.js

+'use strict';
+
+const baseURI = require('document-base-uri');
+
+/**
+ * Return a normalized version of a URI.
+ *
+ * This makes it absolute and strips the fragment identifier.
+ *
+ * @param {string} uri - Relative or absolute URL
+ * @param {string} [base] - Base URL to resolve relative to. Defaults to
+ *   the document's base URL.
+ */
+function normalizeURI(uri, base = baseURI) {
+  const absUrl = new URL(uri, base).href;
+
+  // Remove the fragment identifier.
+  // This is done on the serialized URL rather than modifying `url.hash` due to
+  // a bug in Safari.
+  // See https://github.com/hypothesis/h/issues/3471#issuecomment-226713750
+  return absUrl.toString().replace(/#.*/, '');
+}
+
+module.exports = {
+  normalizeURI,
+};