Add widget to display errors, and report dependency_graph errors correctly

lib/src/codegen/html_codegen.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library dev_compiler.src.codegen.html_codegen;
 
 import 'package:html5lib/dom.dart';
 import 'package:html5lib/parser.dart' show parseFragment;
 import 'package:logging/logging.dart' show Logger;
 
@@ skipping 29 matching lines @@
   });
 
   if (options.outputDart) return '${document.outerHtml}\n';
 
   var libraries = [];
   visitInPostOrder(root, (n) {
     if (n is DartSourceNode) libraries.add(n);
   }, includeParts: false);
 
   String mainLibraryName;
-  var fragment = _loadRuntimeScripts();
+  var fragment = _loadRuntimeScripts(options);
   if (!options.checkSdk) fragment.nodes.add(_miniMockSdk);
   for (var lib in libraries) {
     var info = lib.info;
     if (info == null) continue;
     if (info.isEntry) mainLibraryName = jsLibraryName(info.library);
     fragment.nodes.add(_libraryInclude(jsOutputPath(info, root.uri)));
   }
   fragment.nodes.add(_invokeMain(mainLibraryName));
   scripts[0].replaceWith(fragment);
   return '${document.outerHtml}\n';
 }
 
 /// A document fragment with scripts that check for harmony features and that
 /// inject our runtime.
-Node _loadRuntimeScripts() => parseFragment('''
-<script src="dev_compiler/runtime/harmony_feature_check.js"></script>
-<script src="dev_compiler/runtime/dart_runtime.js"></script>
-''');
+Node _loadRuntimeScripts(options) {
+  // TODO(sigmund): use dev_compiler to generate messages_widget in the future.
+  var widgetCode = options.serverMode
+      ? '<script src="dev_compiler/runtime/messages_widget.js"></script>\n'
+          '<link rel="stylesheet" href="dev_compiler/runtime/messages.css">'
+      : '';
+  return parseFragment(
+      '<script src="dev_compiler/runtime/harmony_feature_check.js"></script>\n'
+      '<script src="dev_compiler/runtime/dart_runtime.js"></script>\n'
+      '$widgetCode');
+}
 
 /// A script tag that loads the .js code for a compiled library.
 Node _libraryInclude(String jsUrl) =>
     parseFragment('<script src="$jsUrl"></script>\n');
 
 /// A script tag that invokes the main function on the entry point library.
-Node _invokeMain(String mainLibraryName) =>
-    parseFragment('<script>$mainLibraryName.main();</script>\n');
+Node _invokeMain(String mainLibraryName) {
+  var code = mainLibraryName == null
+      ? 'console.error("dev_compiler error: main was not generated");'
+      : '$mainLibraryName.main();';
+  return parseFragment('<script>$code</script>\n');
+}
 
 /// A script tag with a tiny mock of the core SDK. This is just used for testing
 /// some small samples.
 // TODO(sigmund,jmesserly): remove.
 Node get _miniMockSdk => parseFragment('''
 <script>
   /* placehorder for unimplemented code libraries */
   var math = Math;
   var core = { int: { parse: Number }, print: e => console.log(e) };
   var dom = { document: document };
 </script>''');
 final _log = new Logger('dev_compiler.src.codegen.html_codegen');