Adding some tests.

src/js/links.js

@@ -91,7 +91,7 @@
       // Filter blocked and pinned links and duplicate base domains.
       links = links.filter(function(link) {
         let site = ProviderManager.extractSite(link.url);
-        if (site === null || sites.has(site)) {
+        if (!site || sites.has(site)) {
           return false;
         }
         sites.add(site);

src/test/directoryLinksProvider_spec.js

@@ -54,4 +54,10 @@ describe("DirectoryLinksProvider API", function() {
   it("should escape chars", function() {
     assert.equal("&lt;test&gt;", DirectoryLinksProvider._escapeChars("<test>"));
   });
+
+  it("should not cache links that don't have frecent_sites", function() {
+    DirectoryLinksProvider._suggestedLinks.clear();
+    DirectoryLinksProvider._cacheSuggestedLinks(directoryLinks.directory[0]);
+    assert.equal(DirectoryLinksProvider._suggestedLinks.size, 0);
+  });
 });

src/test/directoryLinksWithAdgroup.json

@@ -8,6 +8,7 @@
     "directoryId": 2,
     "title": "Private Browsing with Tracking Protection",
     "type": "affiliate",
+    "enhancedImageURI": "pretendThisIsAnImage",
     "url": "https://www.mozilla.org/firefox/private-browsing/?utm_source=directory-tiles&utm_medium=tiles&utm_content=TPV1&utm_campaign=fx-fall-15"
   }],
   "suggested": [{

src/test/links_spec.js

@@ -19,6 +19,13 @@ describe("Links API", function() {
       "url": "http://www.test2.com/",
       "frecency": 101,
       "lastVisitDate": Date.now()
+    },
+    {
+      "title": "Test3",
+      "type": "history",
+      "url": "meep.www.test2.com/",
+      "frecency": 102,
+      "lastVisitDate": Date.now()
     }
   ];
 
@@ -45,8 +52,24 @@ describe("Links API", function() {
     // There are 6 links: 2 directory, 2 places, and 2 pinned
     assert.equal(links.links.length, 6);
 
+    // A forced cache populate should have the same results.
+    yield Links.populateCache({force: true});
+    links = yield Links.getLinks();
+    assert.equal(links.links.length, 6);
+
     // Unpin links.
     yield gPinnedLinks.unpin(firstLink);
     yield gPinnedLinks.unpin(secondLink);
   }));
+
+  it("should throw if links don't have required attributes", function() {
+    expect(() => Links.compareLinks(firstLink, secondLink)).to.throw(Error);
+  });
+
+  it("should not increment site map for blocked links", function() {
+    var m = new Map();
+    gBlockedLinks.block(firstLink);
+    Links._incrementSiteMap(m, firstLink);
+    assert.equal(m.size, 0);
+  });
 });