set isAttached constructor (for closure) but set to undefined so no…

…t picked up as proto property (avoids initial binding value)

closure.log

@@ -5,12 +5,24 @@ Found    : undefined
     return window.Polymer._polymerFn(info);
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+/polymer.html_script_1.js:44: WARNING - The right side in the assignment is not a subtype of the left side.
+Expected : T
+Found    : Function|null
+More details:
+The found type is a union that includes an unexpected type: null
+    mixin = cachingMixin(mixin);
+    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 /polymer.html_script_1.js:44: WARNING - Invalid type for parameter 1 of function cachingMixin.
 Expected : Function|null
 Found    : T
 
-    let cachedMixin = cachingMixin(mixin);
-                                   ^^^^^
+    mixin = cachingMixin(mixin);
+                         ^^^^^
+
+/polymer.html_script_1.js:46: WARNING - Cannot create property __dedupeId on non-object type T.
+    mixin.__dedupeId = ++dedupeId;
+    ^^^^^
 
 /polymer.html_script_1.js:47: WARNING - Returned type does not match declared return type.
 Expected : T
@@ -693,7 +705,7 @@ Found    : PolymerElement
     !(window.customElements.polyfillWrapFlushCallback);
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-/polymer.html_script_22.js:855: WARNING - Function this.splice: called with 3 argument(s). Function requires at least 4 argument(s) and at most 4.
+/polymer.html_script_22.js:857: WARNING - Function this.splice: called with 3 argument(s). Function requires at least 4 argument(s) and at most 4.
             return this.splice(arrayOrPath, index, 1);
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
@@ -1366,4 +1378,4 @@ externs/closure-types.js:759: WARNING - property toggle on interface Polymer_Arr
 Polymer_ArraySelectorMixin.prototype.toggle;
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-0 error(s), 297 warning(s), 72.7% typed
+0 error(s), 299 warning(s), 72.7% typed

lib/legacy/legacy-element-mixin.html

@@ -68,10 +68,12 @@
       constructor() {
         super();
         this.root = this;
-        this.created();
-        this.isAttached = false;
+        // initialize for closure but don't make false or a binding will see
+        // this setting.
+        this.isAttached = undefined;
         this.__boundListeners = null;
         this._debouncers = null;
+        this.created();
       }
 
       /**