Introduce the legium component in test.dart, and start support for dart:dom and dart:html.

frog/leg/native_handler.dart

Index: frog/leg/native_handler.dart
===================================================================
--- frog/leg/native_handler.dart	(revision 4753)
+++ frog/leg/native_handler.dart	(working copy)
@@ -30,8 +30,12 @@
   }
 }
 
-void checkNativeSupport(Compiler compiler, LibraryElement library) {
-  if (library.script.name.contains('dart/frog/tests/native/src')) {
+void checkNativeSupport(Compiler compiler,
+                        LibraryElement library,
+                        [libraryName = '']) {
+  if (library.script.name.contains('dart/frog/tests/native/src')
+      || libraryName == 'dart:dom'
+      || libraryName == 'dart:html') {
     library.define(new ForeignElement(
         const SourceString('native'), library), compiler);
   }