create a unique observer for each ShadowRoot or Document so detached …

…nodes with shadow roots can be collected

src/Observer.js

@@ -246,10 +246,7 @@ function watchShadow(node) {
 }
 
 function watchRoot(root) {
-  if (!root.__watched) {
-    observe(root);
-    root.__watched = true;
-  }
+  observe(root);
 }
 
 function handler(mutations) {
@@ -294,18 +291,32 @@ function handler(mutations) {
   logFlags.dom && console.groupEnd();
 };
 
-var observer = new MutationObserver(handler);
+function takeRecords(node) {
+  // If the optional node is not supplied, assume we mean the whole document.
+  if (node === undefined) node = document;
+
+  // Find the root of the tree, which will be an Document or ShadowRoot.
+  while (node.parentNode) {
+    node = node.parentNode;
+  }
 
-function takeRecords() {
-  // TODO(sjmiles): ask Raf why we have to call handler ourselves
-  handler(observer.takeRecords());
-  takeMutations();
+  var observer = node.__observer;
+  if (observer) {
+    handler(observer.takeRecords());
+    takeMutations();
+  }
 }
 
 var forEach = Array.prototype.forEach.call.bind(Array.prototype.forEach);
 
 function observe(inRoot) {
+  if (inRoot.__observer) return;
+
+  // For each ShadowRoot, we create a new MutationObserver, so the root can be
+  // garbage collected once all references to the `inRoot` node are gone.
+  var observer = new MutationObserver(handler);
   observer.observe(inRoot, {childList: true, subtree: true});
+  inRoot.__observer = observer;
 }
 
 function observeDocument(doc) {