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 da6b0c4ca9176f50daf9bbb3a484a47fd05cdf3f..0fbb0fefa719245f82ec80fe4367dcbedd9e078c 100644
--- a/lib/runtime/dart/html.js
+++ b/lib/runtime/dart/html.js
@@ -12435,7 +12435,7 @@ 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__;
       }