Support source maps in server mode

lib/runtime/dart/html.js

 dart_library.library('dart/html', null, /* Imports */[
   "dart/_runtime",
   'dart/math',
   'dart/core',
   'dart/_js_helper',
   'dart/async',
   'dart/collection',
   'dart/_foreign_helper',
   'dart/isolate'
 ], /* Lazy imports */[
@@ skipping 12528 matching lines @@
       return jso[_wrapper];
     }
     let constructor = jso.constructor;
     let f = null;
     let name = null;
     let skip = false;
     while (f == null) {
       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__;
       }
     }
     if (dart.notNull(skip)) {
       dart.dsend(/* Unimplemented unknown name */console, 'info', `Instantiated ${name} instead`);
     }
     let wrapped = dart.dcall(f);
     dart.dput(wrapped, 'raw', jso);
     jso[_wrapper] = wrapped;
@@ skipping 137 matching lines @@
   exports.ElementUpgrader = ElementUpgrader;
   exports.NodeValidator = NodeValidator;
   exports.NodeTreeSanitizer = NodeTreeSanitizer;
   exports.UriPolicy = UriPolicy;
   exports.spawnDomUri = spawnDomUri;
   exports.unwrap_jso = unwrap_jso;
   exports.wrap_jso = wrap_jso;
   exports.createCustomUpgrader = createCustomUpgrader;
   exports.getHtmlCreateFunction = getHtmlCreateFunction;
 });