[dart:html] Move NullWindowException to implementation

Window.open may open a null window in more cases than expected.
Users may not care that the window they get back is invalid if
they never use it. Therefore, this CL moves the exception to
the implementation of the returned window in order to reduce
noise, but still give a way for users to recover if they wish
to do so.

CoreLibraryReviewExempt: Backend-specific library.
Change-Id: I005cf80630cfb4db2f5ec2012cfcd0161ad10ff1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/311460
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>

CHANGELOG.md

@@ -36,14 +36,6 @@
 [#51486]: https://github.com/dart-lang/sdk/issues/51486
 [#52027]: https://github.com/dart-lang/sdk/issues/52027
 
-#### `dart:html`
-
-- **Breaking change to Window.open**:
-  `Window.open` will now throw an exception that can be caught
-  (`NullWindowException`) if the opened window is null. Previously, this null
-  window would be wrapped, and there would be surprising runtime errors when any
-  member is used on the wrapper.
-
 #### `dart:js_interop`
 
 - **Object literal constructors**:

sdk/lib/html/dart2js/html_dart2js.dart

@@ -32154,8 +32154,6 @@ class Window extends EventTarget
   /**
    * Opens a new window.
    *
-   * Throws a NullWindowException if the opened window is null.
-   *
    * ## Other resources
    *
    * * [Window.open](https://developer.mozilla.org/en-US/docs/Web/API/Window.open)
@@ -32164,7 +32162,6 @@ class Window extends EventTarget
   WindowBase open(String url, String name, [String? options]) {
     final win =
         options == null ? _open2(url, name) : _open3(url, name, options);
-    if (win == null) throw new NullWindowException();
     return _DOMWindowCrossFrame._createSafe(win);
   }
 
@@ -33782,7 +33779,7 @@ class Window extends EventTarget
 class NullWindowException implements Exception {
   @override
   String toString() {
-    return 'Attempted to call Window.open with a null window.';
+    return 'Attempting to use a null window opened in Window.open.';
   }
 }
 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
@@ -40124,7 +40121,12 @@ class _DOMWindowCrossFrame implements WindowBase {
   // Private window.  Note, this is a window in another frame, so it
   // cannot be typed as "Window" as its prototype is not patched
   // properly.  Its fields and methods can only be accessed via JavaScript.
-  final Object _window;
+  final Object? __window;
+
+  Object get _window {
+    if (__window == null) throw new NullWindowException();
+    return __window!;
+  }
 
   // Fields.
   HistoryBase get history =>
@@ -40161,7 +40163,7 @@ class _DOMWindowCrossFrame implements WindowBase {
   }
 
   // Implementation support.
-  _DOMWindowCrossFrame(this._window);
+  _DOMWindowCrossFrame(this.__window);
 
   static WindowBase _createSafe(w) {
     if (identical(w, window)) {

tests/lib/html/window_test.dart

@@ -12,12 +12,15 @@ main() {
     expect(window.scrollY, 0);
   });
   test('open', () {
-    window.open('', 'blank');
+    final valid = window.open('', 'blank');
+    valid.closed;
+    // A blank page with no access to the original window (noopener) should
+    // result in null.
+    final invalid = window.open('', 'invalid', 'noopener=true');
     try {
-      // A blank page with no access to the original window (noopener) should
-      // result in null.
-      window.open('', 'invalid', 'noopener=true');
-      fail('Expected Window.open to throw.');
+      // Should result in an exception since the underlying window is null.
+      invalid.closed;
+      fail('Expected invalid.closed to throw.');
     } on NullWindowException {}
   });
 }

tests/lib_2/html/window_test.dart

@@ -13,4 +13,16 @@ main() {
     expect(window.scrollX, 0);
     expect(window.scrollY, 0);
   });
+  test('open', () {
+    final valid = window.open('', 'blank');
+    valid.closed;
+    // A blank page with no access to the original window (noopener) should
+    // result in null.
+    final invalid = window.open('', 'invalid', 'noopener=true');
+    try {
+      // Should result in an exception since the underlying window is null.
+      invalid.closed;
+      fail('Expected invalid.closed to throw.');
+    } on NullWindowException {}
+  });
 }

tools/dom/src/dart2js_DOMImplementation.dart

@@ -9,7 +9,12 @@ class _DOMWindowCrossFrame implements WindowBase {
   // Private window.  Note, this is a window in another frame, so it
   // cannot be typed as "Window" as its prototype is not patched
   // properly.  Its fields and methods can only be accessed via JavaScript.
-  final Object _window;
+  final Object? __window;
+
+  Object get _window {
+    if (__window == null) throw new NullWindowException();
+    return __window!;
+  }
 
   // Fields.
   HistoryBase get history =>
@@ -46,7 +51,7 @@ class _DOMWindowCrossFrame implements WindowBase {
   }
 
   // Implementation support.
-  _DOMWindowCrossFrame(this._window);
+  _DOMWindowCrossFrame(this.__window);
 
   static WindowBase _createSafe(w) {
     if (identical(w, window)) {

tools/dom/templates/html/impl/impl_Window.darttemplate

@@ -50,8 +50,6 @@ $(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   /**
    * Opens a new window.
    *
-   * Throws a NullWindowException if the opened window is null.
-   *
    * ## Other resources
    *
    * * [Window.open](https://developer.mozilla.org/en-US/docs/Web/API/Window.open)
@@ -60,7 +58,6 @@ $(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   WindowBase open(String url, String name, [String$NULLABLE options]) {
     final win =
         options == null ? _open2(url, name) : _open3(url, name, options);
-    if (win == null) throw new NullWindowException();
     return _DOMWindowCrossFrame._createSafe(win);
   }
 
@@ -259,6 +256,6 @@ $!MEMBERS
 class NullWindowException implements Exception {
   @override
   String toString() {
-    return 'Attempted to call Window.open with a null window.';
+    return 'Attempting to use a null window opened in Window.open.';
   }
 }