Ensure getInstanceModel is only called when a new instance will be created

pkg/mdv/test/binding_syntax_test.dart

Index: pkg/mdv/test/binding_syntax_test.dart
diff --git a/pkg/mdv/test/binding_syntax_test.dart b/pkg/mdv/test/binding_syntax_test.dart
index e94d0af4b639fef876385ffacc9330580684032c..9ef9c68580123378701e06350d013bccc41276d4 100644
--- a/pkg/mdv/test/binding_syntax_test.dart
+++ b/pkg/mdv/test/binding_syntax_test.dart
@@ -97,6 +97,23 @@ syntaxTests(FooBarModel fooModel([foo, bar])) {
     ]);
   });
 
+  observeTest('getInstanceModel - reorder instances', () {
+    var model = toObservable([0, 1, 2]);
+
+    var div = createTestHtml('<template repeat syntax="Test">{{}}</template>');
+    var template = div.firstChild;
+    var delegate = new TestInstanceModelSyntax();
+
+    recursivelySetTemplateModel(div, model, delegate);
+    performMicrotaskCheckpoint();
+    expect(delegate.count, 3);
+
+    // Note: intentionally mutate in place.
+    model.replaceRange(0, model.length, model.reversed.toList());
+    performMicrotaskCheckpoint();
+    expect(delegate.count, 3);
+  });
+
   observeTest('Basic', () {
     var model = fooModel(2, 4);
     var div = createTestHtml(
@@ -134,6 +151,14 @@ class TestModelSyntax extends BindingDelegate {
   }
 }
 
+class TestInstanceModelSyntax extends BindingDelegate {
+  int count = 0;
+  getInstanceModel(template, model) {
+    count++;
+    return model;
+  }
+}
+
 // Note: this isn't a very smart whitespace handler. A smarter one would only
 // trim indentation, not all whitespace.
 // See "trimOrCompact" in the web_ui Pub package.