Reorganize mdv and observe packages

pkg/mdv/lib/src/element.dart

+// Copyright (c) 2013, 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 mdv;
+
+/** Extensions to the [Element] API. */
+class _ElementExtension extends _NodeExtension {
+  _ElementExtension(Element node) : super(node);
+
+  Element get node => super.node;
+
+  Map<String, StreamSubscription> _attributeBindings;
+
+  // TODO(jmesserly): should path be optional, and default to empty path?
+  // It is used that way in at least one path in JS TemplateElement tests
+  // (see "BindImperative" test in original JS code).
+  void bind(String name, model, String path) {
+    if (_attributeBindings == null) {
+      _attributeBindings = new Map<String, StreamSubscription>();
+    }
+
+    node.xtag.attributes.remove(name);
+
+    var changed;
+    if (name.endsWith('?')) {
+      name = name.substring(0, name.length - 1);
+
+      changed = (value) {
+        if (_Bindings._toBoolean(value)) {
+          node.xtag.attributes[name] = '';
+        } else {
+          node.xtag.attributes.remove(name);
+        }
+      };
+    } else {
+      changed = (value) {
+        // TODO(jmesserly): escape value if needed to protect against XSS.
+        // See https://github.com/polymer-project/mdv/issues/58
+        node.xtag.attributes[name] = value == null ? '' : '$value';
+      };
+    }
+
+    unbind(name);
+
+    _attributeBindings[name] =
+        new PathObserver(model, path).bindSync(changed);
+  }
+
+  void unbind(String name) {
+    if (_attributeBindings != null) {
+      var binding = _attributeBindings.remove(name);
+      if (binding != null) binding.cancel();
+    }
+  }
+
+  void unbindAll() {
+    if (_attributeBindings != null) {
+      for (var binding in _attributeBindings.values) {
+        binding.cancel();
+      }
+      _attributeBindings = null;
+    }
+  }
+}