diff --git a/closure.log b/closure.log
index 0d8578eca4..994df1c3d1 100644
--- a/closure.log
+++ b/closure.log
@@ -5,43 +5,11 @@ 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
-
-    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
-Found    : function({__mixinSet:?} (loose)):{__mixinSet:?} (loose)
-
-    return function(base) {
-    ^^^^^^^^^^^^^^^^^^^^^^^
-
-/polymer.html_script_1.js:49: WARNING - Cannot access property __dedupeId of non-object type T.
-      if (baseSet && baseSet[mixin.__dedupeId]) {
-                             ^^^^^
-
-/polymer.html_script_1.js:52: WARNING - Cannot call non-function type T
-      let extended = mixin(base);
-                     ^^^^^^^^^^^
-
-/polymer.html_script_1.js:58: WARNING - Cannot access property __dedupeId of non-object type T.
-      extended.__mixinSet[mixin.__dedupeId] = true;
-                          ^^^^^
+/polymer.html_script_1.js:59: WARNING - invalid cast - the types do not have a common subtype
+from: function(?=):?
+to  : T
+    return /** @type {T} */(dedupingMixin);
+                           ^^^^^^^^^^^^^^^
 
 /polymer.html_script_10.js:144: WARNING - Property name never defined on trigger of type Object
       let triggerPath = trigger.name;
@@ -1378,4 +1346,4 @@ externs/closure-types.js:762: WARNING - property toggle on interface Polymer_Arr
 Polymer_ArraySelectorMixin.prototype.toggle;
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-0 error(s), 299 warning(s), 72.7% typed
+0 error(s), 293 warning(s), 72.8% typed
diff --git a/lib/utils/mixin.html b/lib/utils/mixin.html
index 9ed0237663..b0f8fb1c62 100644
--- a/lib/utils/mixin.html
+++ b/lib/utils/mixin.html
@@ -20,26 +20,14 @@
   let dedupeId = 0;
 
   /**
-   * Given a mixin producing function, memoize applications of mixin to base
-   * @private
-   * @param {Function} mixin Mixin for which to create a caching mixin.
-   * @return {Function} Returns a mixin which when applied multiple times to the
-   * same base will always return the same extended class.
+   * @constructor
+   * @extends {Function}
    */
-  function cachingMixin(mixin) {
-    return function(base) {
-      if (!mixin.__mixinApplications) {
-        mixin.__mixinApplications = new WeakMap();
-      }
-      let map = mixin.__mixinApplications;
-      let application = map.get(base);
-      if (!application) {
-        application = mixin(base);
-        map.set(base, application);
-      }
-      return application;
-    };
-  }
+  function MixinFunction(){}
+  /** @type {(WeakMap | undefined)} */
+  MixinFunction.prototype.__mixinApplications;
+  /** @type {(Object | undefined)} */
+  MixinFunction.prototype.__mixinSet;
 
   /**
    * Wraps an ES6 class expression mixin such that the mixin is only applied
@@ -53,23 +41,34 @@
    *   mixin applications to base
    */
   Polymer.dedupingMixin = function(mixin) {
-    mixin = cachingMixin(mixin);
+    let mixinApplications = /** @type {!MixinFunction} */(mixin).__mixinApplications;
+    if (!mixinApplications) {
+      mixinApplications = new WeakMap();
+      /** @type {!MixinFunction} */(mixin).__mixinApplications = mixinApplications;
+    }
     // maintain a unique id for each mixin
-    mixin.__dedupeId = ++dedupeId;
-    return function(base) {
-      let baseSet = base.__mixinSet;
-      if (baseSet && baseSet[mixin.__dedupeId]) {
+    let mixinDedupeId = dedupeId++;
+    function dedupingMixin(base) {
+      let baseSet = /** @type {!MixinFunction} */(base).__mixinSet;
+      if (baseSet && baseSet[mixinDedupeId]) {
         return base;
       }
-      let extended = mixin(base);
+      let map = mixinApplications;
+      let extended = map.get(base);
+      if (!extended) {
+        extended = /** @type {!Function} */(mixin)(base);
+        map.set(base, extended);
+      }
       // copy inherited mixin set from the extended class, or the base class
       // NOTE: we avoid use of Set here because some browser (IE11)
       // cannot extend a base Set via the constructor.
-      extended.__mixinSet =
-        Object.create(extended.__mixinSet || baseSet || null);
-      extended.__mixinSet[mixin.__dedupeId] = true;
+      let mixinSet = Object.create(/** @type {!MixinFunction} */(extended).__mixinSet || baseSet || null);
+      mixinSet[mixinDedupeId] = true;
+      /** @type {!MixinFunction} */(extended).__mixinSet = mixinSet;
       return extended;
     }
+
+    return /** @type {T} */(dedupingMixin);
   };
 
 })();