[dart:html] rename register to registerElement per spec

tools/dom/templates/html/impl/impl_Document.darttemplate

Index: tools/dom/templates/html/impl/impl_Document.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_Document.darttemplate b/tools/dom/templates/html/impl/impl_Document.darttemplate
index 98df72e69bd06c9d84c0780b75560bbe6cd62924..d96f6c80affc344d5807fd8724549f110704d787 100644
--- a/tools/dom/templates/html/impl/impl_Document.darttemplate
+++ b/tools/dom/templates/html/impl/impl_Document.darttemplate
@@ -48,8 +48,8 @@ $!MEMBERS
   ElementList queryAll(String relativeSelectors) =>
       querySelectorAll(relativeSelectors);
 
-  /// Checks if [register] is supported on the current platform.
-  bool get supportsRegister {
+  /// Checks if [registerElement] is supported on the current platform.
+  bool get supportsRegisterElement {
 $if DART2JS
     return JS('bool', '("registerElement" in #)', this);
 $else
@@ -57,6 +57,10 @@ $else
 $endif
   }
 
+  /// *Deprecated*: use [supportsRegisterElement] instead.
+  @deprecated
+  bool get supportsRegister => supportsRegisterElement;
+
   @DomName('Document.createElement')
   Element createElement(String tagName, [String typeExtension]) {
 $if DART2JS