Implement children for pure XBlocks in edx-platform

This requires fixing the javascript api implementation, and adding
an implementation of get_block to the ModuleSystem api.

However, the implementation is incomplete, due to mismatches between
the expectations of XModule and XBlock.

Also adds tests using the Acid block to make sure that the javascript
and python apis for children are working correctly.

common/test/acceptance/pages/studio/unit.py

@@ -4,7 +4,7 @@ Unit page in Studio
 
 from bok_choy.page_object import PageObject
 from bok_choy.query import SubQuery
-from bok_choy.promise import Promise, EmptyPromise, fulfill
+from bok_choy.promise import EmptyPromise, fulfill
 
 from . import BASE_URL
 
@@ -24,20 +24,32 @@ class UnitPage(PageObject):
         return "{}/unit/{}".format(BASE_URL, self.unit_locator)
 
     def is_browser_on_page(self):
-        return self.is_css_present('body.view-unit')
+        # Wait until all components have been loaded
+        return (
+            self.is_css_present('body.view-unit') and
+            len(self.q(css=Component.BODY_SELECTOR)) == len(self.q(css='{} .xblock-student_view'.format(Component.BODY_SELECTOR)))
+        )
 
-    def component(self, title):
-        return Component(self.browser, self._locator(title))
+    @property
+    def components(self):
+        """
+        Return a list of components loaded on the unit page.
+        """
+        return self.q(css=Component.BODY_SELECTOR).map(lambda el: Component(self.browser, el['data-locator'])).results
 
-    def _locator(self, title):
-        def _check_func():
-            locators = self.q(css=Component.BODY_SELECTOR).filter(
-                SubQuery(css=Component.NAME_SELECTOR).filter(text=title)
-            ).map(lambda el: el['data-locator']).results
-
-            return (len(locators) > 0, locators[0])
-
-        return fulfill(Promise(_check_func, "Found data locator for component"))
+    def edit_draft(self):
+        """
+        Started editing a draft of this unit.
+        """
+        fulfill(EmptyPromise(
+            lambda: self.q(css='.create-draft').present,
+            'Wait for edit draft link to be present'
+        ))
+        self.q(css='.create-draft').click()
+        fulfill(EmptyPromise(
+            lambda: self.q(css='.editing-draft-alert').present,
+            'Wait for draft mode to be activated'
+        ))
 
 
 class Component(PageObject):

common/test/acceptance/pages/xblock/acid.py

@@ -51,6 +51,16 @@ class AcidView(PageObject):
         """
         return self.test_passed('.document-ready-run')
 
+    @property
+    def child_tests_passed(self):
+        """
+        Whether the tests of children passed
+        """
+        return all([
+            self.test_passed('.child-counts-match'),
+            self.test_passed('.child-values-match')
+        ])
+
     def scope_passed(self, scope):
         return all(
             self.test_passed('.scope-storage-test.scope-{} {}'.format(scope, test))