[MD settings] idle load basic and advanced pages

chrome/test/data/webui/settings/settings_main_test.js

Index: chrome/test/data/webui/settings/settings_main_test.js
diff --git a/chrome/test/data/webui/settings/settings_main_test.js b/chrome/test/data/webui/settings/settings_main_test.js
index dc2715111a1ee04541e1ce9ab69edf025dc16151..0e02be927ba144a1da2868a261f40a9dec5f15c9 100644
--- a/chrome/test/data/webui/settings/settings_main_test.js
+++ b/chrome/test/data/webui/settings/settings_main_test.js
@@ -176,7 +176,7 @@ cr.define('settings_main_page', function() {
         assertEquals(
             expectedBasic, page.$$('#basicPage').style.display);
         assertEquals(
-            expectedAdvanced, page.$$('#advancedPage').style.display);
+            expectedAdvanced, page.$.advancedPageTemplate.get().style.display);

[michaelpg, 2017/02/01 07:53:50]

This is confusing: asserting visibility has a side effect of stamping the
advanced page if it isn't stamped yet.

[dpapad, 2017/02/01 18:10:37]

Good catch! We should be querying the DOM directly for #advancedPage like
before, to not have the assertion itself stamping the advanced page div.

[dschuyler, 2017/02/01 21:52:31]

I can see where using templates for dom-if would make this confusing, but this
is different from a dom-if. This template is not controlling visibility (it's
not a dom-if). I believe the usage is correct. The display style is controlled
by the div within the template (that's what is being tested).

[dschuyler, 2017/02/01 21:52:31]

The .get only expands the idle render template. Whether the advanced page is
hidden is determined by a div and not the template. The idle render template is
_not_ for visibility use.

[michaelpg, 2017/02/02 00:00:35]

The existing function assumes <div id="advancedPage"> exists, and will throw an
error if page.$$('#advancedPage') returns null on line 179. Tests using that
function will fail if showAdvancedPage_() returns false.

That may not have been intentional. (If it was, we should outright assert that
the advanced page is stamped.) But it's a check that is being relaxed by the new
function.

[dschuyler, 2017/02/02 02:28:15]

I agree that if that test were desired it should be tested explicitly. (Or a
comment could be made if the unexpected side effect were being relied on).

       }
 
       // TODO(michaelpg): It would be better not to drill into