Adds support for imperatively created elements to `legacyNoObservedAt…

lib/legacy/legacy-element-mixin.js

@@ -101,6 +101,8 @@ export const LegacyElementMixin = dedupingMixin((base) => {
       // NOTE: Inlined for perf from version of DisableUpgradeMixin.
       /** @type {boolean|undefined} */
       this.__isUpgradeDisabled;
+      /** @type {boolean|undefined} */
+      this.__needsAttributesAtConnected;
     }
 
     /**
@@ -196,6 +198,9 @@ export const LegacyElementMixin = dedupingMixin((base) => {
      * @override
      */
     connectedCallback() {
+      if (this.__needsAttributesAtConnected) {
+        this._takeAttributes();
+      }
       // NOTE: Inlined for perf from version of DisableUpgradeMixin.
       if (!this.__isUpgradeDisabled) {
         super.connectedCallback();
@@ -301,11 +306,12 @@ export const LegacyElementMixin = dedupingMixin((base) => {
         this.root = /** @type {HTMLElement} */(this);
         this.created();
         // Pull all attribute values 1x if `legacyNoObservedAttributes` is set.
-        if (legacyNoObservedAttributes && this.hasAttributes()) {
-          const a = this.attributes;
-          for (let i=0, l=a.length; i < l; i++) {
-            const attr = a[i];
-            this.__attributeReaction(attr.name, null, attr.value);
+        if (legacyNoObservedAttributes) {
+          if (this.hasAttributes()) {
+            this._takeAttributes();
+          // Element created from scratch or parser generated
+          } else if (!this.parentNode) {
+            this.__needsAttributesAtConnected = true;
           }
         }
         // Ensure listeners are applied immediately so that they are
@@ -316,6 +322,14 @@ export const LegacyElementMixin = dedupingMixin((base) => {
       }
     }
 
+    _takeAttributes() {
+      const a = this.attributes;
+      for (let i=0, l=a.length; i < l; i++) {
+        const attr = a[i];
+        this.__attributeReaction(attr.name, null, attr.value);
+      }
+    }
+
     /**
      * Called automatically when an element is initializing.
      * Users may override this method to perform class registration time

test/unit/legacy-noattributes.html

@@ -108,6 +108,27 @@
         assert.equal(el.camelCase, 'camelCase');
       });
 
+      test('imperative creation', () => {
+        el = document.createElement('x-attrs');
+        // Create attributes in an unpatched way.
+        const a = document.createAttribute('foo');
+        a.value = 'foo';
+        el.setAttributeNode(a);
+        const b = document.createAttribute('bar');
+        b.value = 'bar';
+        el.setAttributeNode(b);
+        const c = document.createAttribute('camel-case');
+        c.value = 'camelCase';
+        el.setAttributeNode(c);
+        document.body.appendChild(el);
+        assert.equal(el.foo, 'foo');
+        assert.equal(el.$.child1.getAttribute('foo'), 'x-attrs.foo');
+        assert.equal(el.$.child1.foo, "x-attrs.foo");
+        assert.equal(el.$.child1.$.content.textContent, 'x-attrs.foo');
+        assert.equal(el.camelCase, 'camelCase');
+        document.body.removeChild(el);
+      });
+
       test('created called before attributeChanged', () => {
         assert.isTrue(el.wasCreatedInAttributeChanged);
       });