diff --git a/lib/legacy/legacy-data-mixin.html b/lib/legacy/legacy-data-mixin.html
index 0d0804b097..3ef795cd1e 100644
--- a/lib/legacy/legacy-data-mixin.html
+++ b/lib/legacy/legacy-data-mixin.html
@@ -9,8 +9,9 @@
 -->
 
 <link rel="import" href="class.html">
-<link rel="import" href="../polymer.html">
+<link rel="import" href="../../polymer.html">
 <link rel="import" href="../utils/mixin.html">
+<link rel="import" href="../utils/templatize.html">
 
 <script>
 (function() {
@@ -154,6 +155,13 @@
       Polymer.LegacyDataMixin(superClass)
   );
 
+  Polymer.Templatize.mixin = 
+    Polymer.dedupingMixin(superClass => class extends Polymer.LegacyDataMixin(superClass) {
+      get _legacyUndefinedCheck() {
+        return this._methodHost._legacyUndefinedCheck;
+      }
+    });
+
   console.info('LegacyDataMixin will be applied to all legacy elements.\n' +
                'Set `_legacyUndefinedCheck: true` to enable.');
 
diff --git a/lib/utils/templatize.html b/lib/utils/templatize.html
index 460a25530f..f8bd3d64fc 100644
--- a/lib/utils/templatize.html
+++ b/lib/utils/templatize.html
@@ -297,6 +297,10 @@
       // Anonymous class created by the templatize
       let base = options.mutableData ?
         MutableTemplateInstanceBase : TemplateInstanceBase;
+      // Affordance for global mixins onto TemplatizeInstance
+      if (Polymer.Templatize.mixin) {
+        base = Polymer.Templatize.mixin(base);
+      }
       /**
        * @constructor
        * @extends {base}
diff --git a/test/unit/legacy-data.html b/test/unit/legacy-data.html
index 4da340237b..8badb87d47 100644
--- a/test/unit/legacy-data.html
+++ b/test/unit/legacy-data.html
@@ -23,6 +23,9 @@
       <div id="child"
         computed-single="[[computeSingle(inlineSingleDep)]]"
         computed-multi="[[computeMulti(inlineMultiDep1, inlineMultiDep2)]]">
+        <dom-if if>
+          <template><div id="ifChild" computed-multi="[[computeMulti(inlineMultiIfDep1, inlineMultiIfDep2)]]"></div></template>
+        </dom-if>
       </div>
     </template>
     <script>
@@ -40,6 +43,8 @@
             inlineSingleDep: String,
             inlineMultiDep1: String,
             inlineMultiDep2: String,
+            inlineMultiIfDep1: String,
+            inlineMultiIfDep2: String,
             computedSingle: {
               computed: 'computeSingle(computedSingleDep)'
             },
@@ -128,6 +133,18 @@
     </template>
   </test-fixture>
 
+  <test-fixture id="declarative-multi-if-one-computed-inline">
+    <template>
+      <x-data inline-multi-if-dep1="b"></x-data>
+    </template>
+  </test-fixture>
+
+  <test-fixture id="declarative-multi-if-all-computed-inline">
+    <template>
+      <x-data inline-multi-if-dep1="b" inline-multi-if-dep2="c"></x-data>
+    </template>
+  </test-fixture>
+
   <script>
   (function() {
 
@@ -156,6 +173,7 @@
         el._legacyUndefinedCheck = check;
         Object.assign(el, props);
         document.body.appendChild(el);
+        Polymer.flush();
       }
 
       teardown(() => {
@@ -172,6 +190,8 @@
       const inlineSingleDep = 'inlineSingleDep';
       const inlineMultiDep1 = 'inlineMultiDep1';
       const inlineMultiDep2 = 'inlineMultiDep2';
+      const inlineMultiIfDep1 = 'inlineMultiIfDep1';
+      const inlineMultiIfDep2 = 'inlineMultiIfDep2';
 
       suite('check disabled', () => {
         test('no arguments defined', () => {
@@ -240,6 +260,16 @@
           assertEffects({computeMulti: 1});
           assert.equal(el.$.child.computedMulti, '[inlineMultiDep1,inlineMultiDep2]');
         });
+        test('one inline computeMulti argument defined in dom-if', () => {
+          setupElement(false, {inlineMultiIfDep1});
+          assertEffects({computeMulti: 1});
+          assert.equal(el.$$('#ifChild').computedMulti, '[inlineMultiIfDep1,undefined]');
+        });
+        test('all inline computeMulti argument defined in dom-if', () => {
+          setupElement(false, {inlineMultiIfDep1, inlineMultiIfDep2});
+          assertEffects({computeMulti: 1});
+          assert.equal(el.$$('#ifChild').computedMulti, '[inlineMultiIfDep1,inlineMultiIfDep2]');
+        });
       });
 
       suite('warn', () => {
@@ -309,6 +339,16 @@
           assertEffects({computeMulti: 1});
           assert.equal(el.$.child.computedMulti, '[inlineMultiDep1,inlineMultiDep2]');
         });
+        test('one inline computeMulti argument defined in dom-if', () => {
+          setupElement(true, {inlineMultiIfDep1});
+          assertEffects({warn: 1});
+          assert.equal(el.$$('#ifChild').computedMulti, undefined);
+        });
+        test('all inline computeMulti argument defined in dom-if', () => {
+          setupElement(true, {inlineMultiIfDep1, inlineMultiIfDep2});
+          assertEffects({computeMulti: 1});
+          assert.equal(el.$$('#ifChild').computedMulti, '[inlineMultiIfDep1,inlineMultiIfDep2]');
+        });
       });
     });
 
@@ -361,8 +401,21 @@
         });
         test('inline all computeMulti defined', () => {
           el = fixture('declarative-multi-all-computed-inline');
+          assertEffects({computeMulti: 1});
           assert.equal(el.$.child.computedMulti, '[b,c]');
         });
+        test('one inline computeMulti argument defined in dom-if', () => {
+          el = fixture('declarative-multi-if-one-computed-inline');
+          Polymer.flush();
+          assertEffects({computeMulti: 0, warn: 1});
+          assert.equal(el.$$('#ifChild').computedMulti, undefined);
+        });
+        test('all inline computeMulti argument defined in dom-if', () => {
+          el = fixture('declarative-multi-if-all-computed-inline');
+          Polymer.flush();
+          assertEffects({computeMulti: 1});
+          assert.equal(el.$$('#ifChild').computedMulti, '[b,c]');
+        });
       });
     });