Support source maps in server mode

lib/runtime/dart/html.js

Index: lib/runtime/dart/html.js
diff --git a/lib/runtime/dart/html.js b/lib/runtime/dart/html.js
index f8b5a06a81188a5b7842edf83080ba2d9b513b7f..f7ce0e8277d641d19c77677b224c29c5a645d9fa 100644
--- a/lib/runtime/dart/html.js
+++ b/lib/runtime/dart/html.js
@@ -12546,7 +12546,7 @@ dart_library.library('dart/html', null, /* Imports */[
       name = constructor.name;
       f = getHtmlCreateFunction(name);
       if (f == null) {
-        dart.dsend(/* Unimplemented unknown name */console, 'error', `Could not instantiate ${name}`);
+        dart.dsend(/* Unimplemented unknown name */console, 'warn', `Could not instantiate ${name}`);
         skip = true;
         constructor = constructor.__proto__;
       }