Revert "update platform.js to get memory leak fixes"

pkg/web_components/lib/platform.concat.js

Index: pkg/web_components/lib/platform.concat.js
diff --git a/pkg/web_components/lib/platform.concat.js b/pkg/web_components/lib/platform.concat.js
index c78254b965db32c4aeb1b5d63c2925d52f559cb2..dccbcc9246c1c8a4aaf3786ca2ea16b3a480bba5 100644
--- a/pkg/web_components/lib/platform.concat.js
+++ b/pkg/web_components/lib/platform.concat.js
@@ -2317,13 +2317,7 @@ window.ShadowDOMPolyfill = {};
   var globalMutationObservers = [];
   var isScheduled = false;
 
-  function scheduleCallback(observer) {
-    if (observer.scheduled_)
-      return;
-
-    observer.scheduled_ = true;
-    globalMutationObservers.push(observer);
-
+  function scheduleCallback() {
     if (isScheduled)
       return;
     setEndOfMicrotask(notifyObservers);
@@ -2343,7 +2337,6 @@ window.ShadowDOMPolyfill = {};
 
       for (var i = 0; i < notifyList.length; i++) {
         var mo = notifyList[i];
-        mo.scheduled_ = false;
         var queue = mo.takeRecords();
         removeTransientObserversFor(mo);
         if (queue.length) {
@@ -2459,6 +2452,8 @@ window.ShadowDOMPolyfill = {};
       }
     }
 
+    var anyObserversEnqueued = false;
+
     // 4.
     for (var uid in interestedObservers) {
       var observer = interestedObservers[uid];
@@ -2491,9 +2486,15 @@ window.ShadowDOMPolyfill = {};
         record.oldValue = associatedStrings[uid];
 
       // 8.
-      scheduleCallback(observer);
+      if (!observer.records_.length) {
+        globalMutationObservers.push(observer);
+        anyObserversEnqueued = true;
+      }
       observer.records_.push(record);
     }
+
+    if (anyObserversEnqueued)
+      scheduleCallback();
   }
 
   var slice = Array.prototype.slice;
@@ -2556,7 +2557,6 @@ window.ShadowDOMPolyfill = {};
     this.nodes_ = [];
     this.records_ = [];
     this.uid_ = ++uidCounter;
-    this.scheduled_ = false;
   }
 
   MutationObserver.prototype = {
@@ -2642,10 +2642,6 @@ window.ShadowDOMPolyfill = {};
       if (node === this.target)
         return;
 
-      // Make sure we remove transient observers at the end of microtask, even
-      // if we didn't get any change records.
-      scheduleCallback(this.observer);
-
       this.transientObservedNodes.push(node);
       var registrations = registrationsTable.get(node);
       if (!registrations)
@@ -11377,7 +11373,10 @@ function watchShadow(node) {
 }
 
 function watchRoot(root) {
-  observe(root);
+  if (!root.__watched) {
+    observe(root);
+    root.__watched = true;
+  }
 }
 
 function handler(mutations) {
@@ -11422,32 +11421,18 @@ function handler(mutations) {
   logFlags.dom && console.groupEnd();
 };
 
-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;
-  }
+var observer = new MutationObserver(handler);
 
-  var observer = node.__observer;
-  if (observer) {
-    handler(observer.takeRecords());
-    takeMutations();
-  }
+function takeRecords() {
+  // TODO(sjmiles): ask Raf why we have to call handler ourselves
+  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) {