[dart2js] Fix native null assertions for Dart 3

Allows the following semantics:
- If the disable flag is passed, never check
- If we're in non-sound mode, never check to avoid a breaking
change (this was the behavior before)
- If the enable flag was passed and we're in sound mode,
always check
- If we're in sound mode and with >= -03, don't check

Discussion in https://github.com/dart-lang/sdk/issues/50710

Change-Id: If83aaebb2745e4c8bcaa3f5a38ff41d79a869b8b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289451
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Srujan Gaddam <srujzs@google.com>

CHANGELOG.md

@@ -194,6 +194,19 @@
 
 #### Web Dev Compiler (DDC)
 - Removed deprecated command line flags `-k`, `--kernel`, and `--dart-sdk`.
+- The compile time flag `--nativeNonNullAsserts`, which ensures web library APIs
+are sound in their nullability, is by default set to true in sound mode. For
+more information on the flag, see [NATIVE_NULL_ASSERTIONS.md][].
+
+[NATIVE_NULL_ASSERTIONS.md]: https://github.com/dart-lang/sdk/blob/main/sdk/lib/html/doc/NATIVE_NULL_ASSERTIONS.md
+
+#### dart2js
+- The compile time flag `--native-null-assertions`, which ensures web library
+APIs are sound in their nullability, is by default set to true in sound mode,
+unless `-O3` or higher is passed, in which case they are not checked. For more
+information on the flag, see [NATIVE_NULL_ASSERTIONS.md][].
+
+[NATIVE_NULL_ASSERTIONS.md]: https://github.com/dart-lang/sdk/blob/main/sdk/lib/html/doc/NATIVE_NULL_ASSERTIONS.md
 
 #### Dart2js
 

pkg/compiler/lib/src/dart2js.dart

@@ -1287,6 +1287,11 @@ Usage: dart compile js [arguments] <dart entry point>
     dart2js to generate smaller code by removing late variable names from the
     generated JavaScript.
 
+  --native-null-assertions
+    Add assertions to web library APIs to ensure that non-nullable APIs do not
+    return null. This is by default set to true in sound null-safety, unless
+    -O3 or higher is passed.
+
   -O<0,1,2,3,4>
     Controls optimizations that can help reduce code-size and improve
     performance of the generated code for deployment.

pkg/compiler/lib/src/options.dart

@@ -832,8 +832,21 @@ class CompilerOptions implements DiagnosticOptions {
       omitLateNames = false;
     }
 
-    if (_noNativeNullAssertions || nullSafetyMode != NullSafetyMode.sound) {
+    if (nullSafetyMode != NullSafetyMode.sound) {
+      // Technically, we should still assert if the user passed in a flag to
+      // assert, but this was not the behavior before, so to avoid a breaking
+      // change, we don't assert in unsound mode.
       nativeNullAssertions = false;
+    } else if (_noNativeNullAssertions) {
+      // Never assert if the user tells us not to.
+      nativeNullAssertions = false;
+    } else if (!nativeNullAssertions &&
+        (optimizationLevel != null && optimizationLevel! >= 3)) {
+      // If the user didn't tell us to assert and we're in >= -O3, optimize away
+      // the check. This should reduce issues in production.
+      nativeNullAssertions = false;
+    } else {
+      nativeNullAssertions = true;
     }
 
     if (_mergeFragmentsThreshold != null) {

sdk/lib/html/doc/NATIVE_NULL_ASSERTIONS.md

@@ -37,6 +37,15 @@ Native null assertions will be turned on by default across different build
 systems. If it is enabled, here's how you can disable it in the following build
 systems:
 
+### dart2js
+
+Pass the `--no-native-null-assertions` flag to dart2js. Note that in dart2js,
+we turn off native null assertions when `-O3` or higher is specified.
+
+### DDC
+
+Set `nativeNonNullAsserts` to false.
+
 ### build_web_compilers
 
 https://github.com/dart-lang/build/tree/master/docs/native_null_assertions.md

tests/web/native/native_null_assertions/flag_enabled_optimized_web_test.dart

@@ -0,0 +1,16 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// Requirements=nnbd-strong
+// dart2jsOptions=-O3 --native-null-assertions
+
+import 'null_assertions_test_lib.dart';
+import 'web_library_interfaces.dart';
+
+void main() {
+  // If the user explicitly passes the flag, we can't optimize the check away.
+  var flagEnabled = true;
+  testNativeNullAssertions(flagEnabled);
+  testJSInvocationNullAssertions(flagEnabled);
+}

tests/web/native/native_null_assertions/flag_enabled_unsound_web_test.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// Requirements=nnbd-weak
+// dart2jsOptions=--native-null-assertions
+
+import 'null_assertions_test_lib.dart';
+import 'web_library_interfaces.dart';
+
+void main() {
+  // To avoid a breaking change, we don't enable checks even if the user passes
+  // the flag in unsound mode.
+  var flagEnabled = false;
+  testNativeNullAssertions(flagEnabled);
+  testJSInvocationNullAssertions(flagEnabled);
+}

tests/web/native/native_null_assertions/no_flag_optimized_web_test.dart

@@ -0,0 +1,16 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// Requirements=nnbd-strong
+// dart2jsOptions=-O3
+
+import 'null_assertions_test_lib.dart';
+import 'web_library_interfaces.dart';
+
+void main() {
+  // If the flag wasn't passed, we can optimize the check away.
+  var flagEnabled = false;
+  testNativeNullAssertions(flagEnabled);
+  testJSInvocationNullAssertions(flagEnabled);
+}

tests/web/native/native_null_assertions/no_flag_web_test.dart

@@ -0,0 +1,17 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+// Requirements=nnbd-strong
+// dart2jsOptions=-O1
+
+import 'null_assertions_test_lib.dart';
+import 'web_library_interfaces.dart';
+
+void main() {
+  // Strong mode should enable checks in the default optimization level if
+  // neither the enable or disable flag is passed.
+  var flagEnabled = true;
+  testNativeNullAssertions(flagEnabled);
+  testJSInvocationNullAssertions(flagEnabled);
+}