Updating custom element's preregister test to follow new contracts.

tests/html/html.status

Index: tests/html/html.status
diff --git a/tests/html/html.status b/tests/html/html.status
index 45fda94bc06a2486c0f3f2e86da818a99b3aab1d..8cdf46deb85705046e8f1f6c3c617b70d5a1280b 100644
--- a/tests/html/html.status
+++ b/tests/html/html.status
@@ -9,13 +9,6 @@ interactive_test: Skip # Must be run manually.
 [ $compiler == dart2js && $runtime != drt ]
 custom_elements_test: Skip
 
-[ $compiler == dart2js && $runtime == drt ]
-custom_elements_test/preregister: Fail  # Unclear what the preregister semantics should be.
-
-[ $compiler == dartanalyzer || $compiler == dart2analyzer ]
-# Document.register is unimplemented.
-custom_elements_test: Skip
-
 [ $compiler == none && ($runtime == drt || $runtime == dartium) && $mode == debug && $system == macos]
 audiobuffersourcenode_test: Pass, Fail, Crash # http://crbug.com/256601
 
@@ -23,7 +16,6 @@ audiobuffersourcenode_test: Pass, Fail, Crash # http://crbug.com/256601
 # postMessage in dartium always transfers the typed array buffer, never a view
 postmessage_structured_test/typed_arrays: Fail
 events_test: Pass, Fail # Issue 12489
-custom_elements_test/preregister: Fail # Issue 9326 - Implement "extends Element" in dartium
 custom_elements_test/lifecycle: Fail   # Issue 9326 - Implement "extends Element" in dartium
 
 [ $compiler == none && $runtime == drt && $system == windows ]