Support upgrading existing elements already on the page.

pkg/web_components/test/interop_test.dart

Index: pkg/web_components/test/interop_test.dart
diff --git a/pkg/web_components/test/interop_test.dart b/pkg/web_components/test/interop_test.dart
index 64dfb6426f7bc673fb130653e803f77640f12390..cbad6e192e91d8830eb843ce0acdfc12aaabd722 100644
--- a/pkg/web_components/test/interop_test.dart
+++ b/pkg/web_components/test/interop_test.dart
@@ -12,15 +12,9 @@ import 'package:unittest/unittest.dart';
 import 'package:web_components/interop.dart';
 import 'package:web_components/polyfill.dart';
 
-final globalSetup = customElementsReady.then((_) {
-    registerDartType('x-a', XAWrapper);
-    registerDartType('x-b', XBWrapper, extendsTag: 'div');
-    registerDartType('x-c', XCWrapper);
-  });
-
 main() {
   useHtmlConfiguration();
-  setUp(() => globalSetup);
+  setUp(() => customElementsReady);
 
   test('interop is supported', () {
     expect(isSupported, isTrue);
@@ -37,21 +31,53 @@ main() {
     expect(b is XBWrapper, isFalse, reason: 'x-b should not be upgraded yet');
     expect(_readX(b), 1);
 
+    var d = document.querySelector('x-d');
+    expect(d is HtmlElement, isTrue, reason: 'x-d is HtmlElement');
+    expect(d is XDWrapper, isFalse, reason: 'x-d should not be upgraded yet');
+    expect(_readX(d), 2);
+
+    /// Note: this registration has a global side-effect and is assumed in the
+    /// following tests.
+    registerDartType('x-a', XAWrapper);
+    registerDartType('x-b', XBWrapper, extendsTag: 'div');
+    registerDartType('x-c', XCWrapper);
+    onlyUpgradeNewElements();
+    registerDartType('x-d', XDWrapper); // late on purpose.
+
+    a = document.querySelector('x-a');
+    expect(a is HtmlElement, isTrue, reason: 'x-a is HtmlElement');
+    expect(a is XAWrapper, isTrue, reason: 'x-a is upgraded to XAWrapper');
+    expect(a.x, 0);
+    expect(a.wrapperCount, 0);
+
+    b = document.querySelector('[is=x-b]');
+    expect(b is DivElement, isTrue, reason: 'x-b is DivElement');
+    expect(b is XBWrapper, isTrue, reason: 'x-b is upgraded to XBWrapper');
+    expect(b.x, 1);
+    expect(b.wrapperCount, 1);
+
+    // x-d was not upgraded because its registration came after we stopped
+    // upgrading old elements:
+    d = document.querySelector('x-d');
+    expect(d is HtmlElement, isTrue, reason: 'x-d is HtmlElement');
+    expect(d is XDWrapper, isFalse, reason: 'x-d should not be upgraded yet');
+    expect(_readX(d), 2);
+
     var c = document.querySelector('x-c');
     expect(c is HtmlElement, isTrue, reason: 'x-c is HtmlElement');
     expect(c is XCWrapper, isFalse, reason: 'x-c should not be upgraded yet');
     expect(_readX(c), null, reason: 'x-c has not been registered in JS yet');
   });
 
-  test('events seen for anything created after registering Dart type', () {
+  test('anything created after registering Dart type is upgraded', () {
     context.callMethod('addA');
     var list = document.querySelectorAll('x-a');
     expect(list.length, 2);
     var a = list[1];
     expect(a is HtmlElement, isTrue, reason: 'x-a is HtmlElement');
     expect(a is XAWrapper, isTrue, reason: 'x-a is upgraded to XAWrapper');
-    expect(a.x, 2);
-    expect(a.wrapperCount, 0);
+    expect(a.x, 3);
+    expect(a.wrapperCount, 2);
 
     context.callMethod('addB');
     list = document.querySelectorAll('[is=x-b]');
@@ -59,8 +85,18 @@ main() {
     var b = list[1];
     expect(b is DivElement, isTrue, reason: 'x-b is DivElement');
     expect(b is XBWrapper, isTrue, reason: 'x-b is upgraded to XBWrapper');
-    expect(b.x, 3);
-    expect(b.wrapperCount, 1);
+    expect(b.x, 4);
+    expect(b.wrapperCount, 3);
+
+    // New instances of x-d should be upgraded regardless.
+    context.callMethod('addD');
+    list = document.querySelectorAll('x-d');
+    expect(list.length, 2);
+    var d = list[1];
+    expect(d is HtmlElement, isTrue, reason: 'x-d is HtmlElement');
+    expect(d is XDWrapper, isTrue, reason: 'x-d is upgraded to XDWrapper');
+    expect(d.x, 5);
+    expect(d.wrapperCount, 4);
   });
 
   test('events seen if Dart type is registered before registerElement', () {
@@ -71,8 +107,8 @@ main() {
     context.callMethod('registerC');
     c = document.querySelector('x-c');
     expect(c is XCWrapper, isTrue);
-    expect(c.x, 4);
-    expect(c.wrapperCount, 2);
+    expect(c.x, 6);
+    expect(c.wrapperCount, 5);
 
     context.callMethod('addC');
     var list = document.querySelectorAll('x-c');
@@ -81,8 +117,8 @@ main() {
     c = list[1];
     expect(c is HtmlElement, isTrue, reason: 'x-c is HtmlElement');
     expect(c is XCWrapper, isTrue, reason: 'x-c is upgraded to XCWrapper');
-    expect(c.x, 5);
-    expect(c.wrapperCount, 3);
+    expect(c.x, 7);
+    expect(c.wrapperCount, 6);
   });
 }
 int _count = 0;
@@ -105,3 +141,7 @@ class XBWrapper extends DivElement with Wrapper {
 class XCWrapper extends HtmlElement with Wrapper {
   XCWrapper.created() : super.created();
 }
+
+class XDWrapper extends HtmlElement with Wrapper {
+  XDWrapper.created() : super.created();
+}