dart2js: Adds support for custom elements in CPS.

tests/html/html.status

Index: tests/html/html.status
diff --git a/tests/html/html.status b/tests/html/html.status
index 328e3051cb5ea7910d5d3cf96b947944c31de749..5b30a0a9401d0da6f471b2db1efaf3817be467fd 100644
--- a/tests/html/html.status
+++ b/tests/html/html.status
@@ -420,17 +420,6 @@ window_nosuchmethod_test: StaticWarning
 js_typed_interop_default_arg_test/default_value: MissingCompileTimeError # Issue #25759
 
 [ $compiler == dart2js && $cps_ir && $browser ]
-# Custom element support:
-custom/constructor_calls_created_synchronously_test: RuntimeError # Need custom element support #25484
-custom/element_upgrade_test: RuntimeError # Need custom element support #25484
-custom/mirrors_test: RuntimeError # Need custom element support #25484
-custom_elements_23127_test/baseline: RuntimeError # Need custom element support #25484
-custom_elements_23127_test/c2: RuntimeError # Need custom element support #25484
-custom_elements_23127_test/c1t: RuntimeError # Need custom element support #25484
-custom_elements_23127_test/c2t: RuntimeError # Need custom element support #25484
-custom_elements_test/innerHtml: RuntimeError # Need custom element support #25484
-custom_elements_test/register: RuntimeError # Need custom element support #25484
-
 js_typed_interop_side_cast_exp_test: RuntimeError # Corner case in package:js that we might want to remove (See comment in #24978).
 js_typed_interop_test/static_method_tearoff_1: RuntimeError # Tree-shaking a used tear-off (#24978, #25720).
 js_typed_interop_default_arg_test/explicit_argument: RuntimeError # Tree-shaking a used tear-off (#24978, #25720).