Add back in null assertions for legacy weak mode

These assertions need to be added back into the nnbd sdk in order to be
compatible with weak mode.

Change-Id: Ie774fffda1fb0e79dbc7fe13cb75b9d28e4268fd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/136720
Commit-Queue: Srujan Gaddam <srujzs@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Stephen Adams <sra@google.com>

sdk_nnbd/lib/html/dart2js/html_dart2js.dart

@@ -10342,13 +10342,13 @@ class DocumentFragment extends Node
   // Native field is used only by Dart code so does not lead to instantiation
   // of native classes
   @Creates('Null')
-  List<Element> _docChildren;
+  List<Element>? _docChildren;
 
   List<Element> get children {
     if (_docChildren == null) {
       _docChildren = new FilteredElementList(this);
     }
-    return _docChildren;
+    return _docChildren!;
   }
 
   set children(List<Element> value) {
@@ -12814,6 +12814,7 @@ class Element extends Node
   @pragma('dart2js:tryInline')
   String? getAttribute(String name) {
     // Protect [name] against string conversion to "null" or "undefined".
+    assert(name != null, 'Attribute name cannot be null');
     return _getAttribute(name);
   }
 
@@ -12821,12 +12822,14 @@ class Element extends Node
   String? getAttributeNS(String? namespaceURI, String name) {
     // Protect [name] against string conversion to "null" or "undefined".
     // [namespaceURI] does not need protecting, both `null` and `undefined` map to `null`.
+    assert(name != null, 'Attribute name cannot be null');
     return _getAttributeNS(namespaceURI, name);
   }
 
   @pragma('dart2js:tryInline')
   bool hasAttribute(String name) {
     // Protect [name] against string conversion to "null" or "undefined".
+    assert(name != null, 'Attribute name cannot be null');
     return _hasAttribute(name);
   }
 
@@ -12834,30 +12837,37 @@ class Element extends Node
   bool hasAttributeNS(String? namespaceURI, String name) {
     // Protect [name] against string conversion to "null" or "undefined".
     // [namespaceURI] does not need protecting, both `null` and `undefined` map to `null`.
+    assert(name != null, 'Attribute name cannot be null');
     return _hasAttributeNS(namespaceURI, name);
   }
 
   @pragma('dart2js:tryInline')
   void removeAttribute(String name) {
     // Protect [name] against string conversion to "null" or "undefined".
+    assert(name != null, 'Attribute name cannot be null');
     _removeAttribute(name);
   }
 
   @pragma('dart2js:tryInline')
   void removeAttributeNS(String? namespaceURI, String name) {
     // Protect [name] against string conversion to "null" or "undefined".
+    assert(name != null, 'Attribute name cannot be null');
     _removeAttributeNS(namespaceURI, name);
   }
 
   @pragma('dart2js:tryInline')
   void setAttribute(String name, String value) {
     // Protect [name] against string conversion to "null" or "undefined".
+    assert(name != null, 'Attribute name cannot be null');
+    // TODO(sra): assert(value != null, 'Attribute value cannot be null.');
     _setAttribute(name, value);
   }
 
   @pragma('dart2js:tryInline')
   void setAttributeNS(String? namespaceURI, String name, String value) {
     // Protect [name] against string conversion to "null" or "undefined".
+    assert(name != null, 'Attribute name cannot be null');
+    // TODO(sra): assert(value != null, 'Attribute value cannot be null.');
     _setAttributeNS(namespaceURI, name, value);
   }
 
@@ -32590,7 +32600,7 @@ class Window extends EventTarget
    */
   int requestAnimationFrame(FrameRequestCallback callback) {
     _ensureRequestAnimationFrame();
-    return _requestAnimationFrame(_wrapZone(callback));
+    return _requestAnimationFrame(_wrapZone(callback)!);
   }
 
   /**
@@ -41233,15 +41243,17 @@ class _WrappedEvent implements Event {
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-void Function(T) _wrapZone<T>(void Function(T) callback) {
+void Function(T)? _wrapZone<T>(void Function(T) callback) {
   // For performance reasons avoid wrapping if we are in the root zone.
   if (Zone.current == Zone.root) return callback;
+  if (callback == null) return null;
   return Zone.current.bindUnaryCallbackGuarded(callback);
 }
 
-void Function(T1, T2) _wrapBinaryZone<T1, T2>(void Function(T1, T2) callback) {
+void Function(T1, T2)? _wrapBinaryZone<T1, T2>(void Function(T1, T2) callback) {
   // For performance reasons avoid wrapping if we are in the root zone.
   if (Zone.current == Zone.root) return callback;
+  if (callback == null) return null;
   return Zone.current.bindBinaryCallbackGuarded(callback);
 }
 

tools/dom/nnbd_src/shared_html.dart

@@ -4,15 +4,17 @@
 
 part of dart.dom.html;
 
-void Function(T) _wrapZone<T>(void Function(T) callback) {
+void Function(T)? _wrapZone<T>(void Function(T) callback) {
   // For performance reasons avoid wrapping if we are in the root zone.
   if (Zone.current == Zone.root) return callback;
+  if (callback == null) return null;
   return Zone.current.bindUnaryCallbackGuarded(callback);
 }
 
-void Function(T1, T2) _wrapBinaryZone<T1, T2>(void Function(T1, T2) callback) {
+void Function(T1, T2)? _wrapBinaryZone<T1, T2>(void Function(T1, T2) callback) {
   // For performance reasons avoid wrapping if we are in the root zone.
   if (Zone.current == Zone.root) return callback;
+  if (callback == null) return null;
   return Zone.current.bindBinaryCallbackGuarded(callback);
 }
 

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

@@ -29,13 +29,13 @@ $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS
   // Native field is used only by Dart code so does not lead to instantiation
   // of native classes
   @Creates('Null')
-  List<Element> _docChildren;
+  List<Element>$NULLABLE _docChildren;
 
   List<Element> get children {
     if (_docChildren == null) {
       _docChildren = new FilteredElementList(this);
     }
-    return _docChildren;
+    return _docChildren$NULLASSERT;
   }
 
   set children(List<Element> value) {

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

@@ -567,10 +567,7 @@ $(ANNOTATIONS)$(NATIVESPEC)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   @pragma('dart2js:tryInline')
   String$NULLABLE getAttribute(String name) {
     // Protect [name] against string conversion to "null" or "undefined".
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
-$endif
     return _getAttribute(name);
   }
 
@@ -578,20 +575,14 @@ $endif
   String$NULLABLE getAttributeNS(String$NULLABLE namespaceURI, String name) {
     // Protect [name] against string conversion to "null" or "undefined".
     // [namespaceURI] does not need protecting, both `null` and `undefined` map to `null`.
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
-$endif
     return _getAttributeNS(namespaceURI, name);
   }
 
   @pragma('dart2js:tryInline')
   bool hasAttribute(String name) {
     // Protect [name] against string conversion to "null" or "undefined".
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
-$endif
     return _hasAttribute(name);
   }
 
@@ -599,52 +590,37 @@ $endif
   bool hasAttributeNS(String$NULLABLE namespaceURI, String name) {
     // Protect [name] against string conversion to "null" or "undefined".
     // [namespaceURI] does not need protecting, both `null` and `undefined` map to `null`.
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
-$endif
     return _hasAttributeNS(namespaceURI, name);
   }
 
   @pragma('dart2js:tryInline')
   void removeAttribute(String name) {
     // Protect [name] against string conversion to "null" or "undefined".
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
-$endif
     _removeAttribute(name);
   }
 
   @pragma('dart2js:tryInline')
   void removeAttributeNS(String$NULLABLE namespaceURI, String name) {
     // Protect [name] against string conversion to "null" or "undefined".
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
-$endif
     _removeAttributeNS(namespaceURI, name);
   }
 
   @pragma('dart2js:tryInline')
   void setAttribute(String name, String value) {
     // Protect [name] against string conversion to "null" or "undefined".
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
     // TODO(sra): assert(value != null, 'Attribute value cannot be null.');
-$endif
     _setAttribute(name, value);
   }
 
   @pragma('dart2js:tryInline')
   void setAttributeNS(String$NULLABLE namespaceURI, String name, String value) {
     // Protect [name] against string conversion to "null" or "undefined".
-$if NNBD
-$else
     assert(name != null, 'Attribute name cannot be null');
     // TODO(sra): assert(value != null, 'Attribute value cannot be null.');
-$endif
     _setAttributeNS(namespaceURI, name, value);
   }
 

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

@@ -106,7 +106,7 @@ $(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
    */
   int requestAnimationFrame(FrameRequestCallback callback) {
     _ensureRequestAnimationFrame();
-    return _requestAnimationFrame(_wrapZone(callback));
+    return _requestAnimationFrame(_wrapZone(callback)$NULLASSERT);
   }
 
   /**