Add a warning about using a component that isn't referenced

lib/src/analyzer.dart

 // Copyright (c) 2012, 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.
 
 /**
  * Part of the template compilation that concerns with extracting information
  * from the HTML parse tree.
  */
 library analyzer;
 
@@ skipping 117 matching lines @@
     }
 
     _bindCustomElement(node, info);
 
     var lastInfo = _currentInfo;
     if (node.tagName == 'element') {
       // If element is invalid _ElementLoader already reported an error, but
       // we skip the body of the element here.
       var name = node.attributes['name'];
       if (name == null) return;
+
       var component = _fileInfo.components[name];
       if (component == null) return;
 
       // Associate ElementInfo of the <element> tag with its component.
       component.elemInfo = info;
 
       _bindExtends(component);
 
       _currentInfo = component;
     }
@@ skipping 46 matching lines @@
 
     return info.hasDataBinding || info.hasIfCondition || info.hasIterate
        || info.hasQuery || info.component != null || info.values.length > 0 ||
        info.events.length > 0;
   }
 
   void _bindExtends(ComponentInfo component) {
     component.extendsComponent = _fileInfo.components[component.extendsTag];
     if (component.extendsComponent == null &&
         component.extendsTag.startsWith('x-')) {
-
       _messages.warning(
           'custom element with tag name ${component.extendsTag} not found.',
           component.element.sourceSpan, file: _fileInfo.path);
     }
   }
 
   void _bindCustomElement(Element node, ElementInfo info) {
     // <x-fancy-button>
     var component = _fileInfo.components[node.tagName];
     if (component == null) {
       // TODO(jmesserly): warn for unknown element tags?
 
       // <button is="x-fancy-button">
-      var isAttr = node.attributes['is'];
-      if (isAttr != null) {
-        component = _fileInfo.components[isAttr];
-        if (component == null) {
-          _messages.warning('custom element with tag name $isAttr not found.',
-              node.sourceSpan, file: _fileInfo.path);
-        }
+      var componentName = node.attributes['is'];
+      if (componentName != null) {
+        component = _fileInfo.components[componentName];
+      } else if (node.tagName.startsWith('x-')) {
+        componentName = node.tagName;
+      }
+      if (component == null && componentName != null) {
+        _messages.warning(
+            'custom element with tag name $componentName not found.',
+            node.sourceSpan, file: _fileInfo.path);
       }
     }
 
     if (component != null && !component.hasConflict) {
       info.component = component;
       _currentInfo.usedComponents[component] = true;
     }
   }
 
   TemplateInfo _createTemplateInfo(Element node) {
@@ skipping 719 matching lines @@
     end = text.indexOf('}}', start);
     if (end < 0) {
       start = length;
       return false;
     }
     // For consistency, start and end both include the curly braces.
     end += 2;
     return true;
   }
 }