Add supported checks to the SVG library, and library cleanup.

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

Index: tools/dom/templates/html/impl/impl_SVGElement.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_SVGElement.darttemplate b/tools/dom/templates/html/impl/impl_SVGElement.darttemplate
index 42ad4508420ae38b99b7b98f44c59ad06890197f..86c34093a9077b676620bcd1806e1d91ac5fc451 100644
--- a/tools/dom/templates/html/impl/impl_SVGElement.darttemplate
+++ b/tools/dom/templates/html/impl/impl_SVGElement.darttemplate
@@ -111,5 +111,14 @@ $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
     throw new UnsupportedError("Cannot invoke click SVG.");
   }
 
+  /**
+   * Checks to see if the SVG element type is supported by the current platform.
+   *
+   * The tag should be a valid SVG element tag name.
+   */
+  static bool isTagSupported(String tag) {
+    var e = new $CLASSNAME.tag(tag);
+    return e is $CLASSNAME && !(e is UnknownElement);
+  }
 $!MEMBERS
 }