diff --git a/CHANGELOG.md b/CHANGELOG.md
index 443724ac352..8ce02c18969 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,15 @@
 ## 3.9.0
 
+### Language
+
+Dart 3.9 assumes null safety when computing type promotion, reachability, and
+definite assignment. This makes these features produce more accurate results for
+modern Dart programs. As a result of this change, more dead_code warnings may be
+produced. To take advantage of these improvements, set your package's [SDK
+constraint][language version] lower bound to 3.9 or greater (`sdk: '^3.9.0'`).
+
+[language version]: https://dart.dev/guides/language/evolution
+
 ### Tools
 
 #### Dart Development Compiler (dartdevc)
diff --git a/pkg/_fe_analyzer_shared/lib/src/experiments/flags.dart b/pkg/_fe_analyzer_shared/lib/src/experiments/flags.dart
index c4c3db64351..43563f2149b 100644
--- a/pkg/_fe_analyzer_shared/lib/src/experiments/flags.dart
+++ b/pkg/_fe_analyzer_shared/lib/src/experiments/flags.dart
@@ -222,10 +222,10 @@ enum ExperimentalFlag {
 
   soundFlowAnalysis(
       name: 'sound-flow-analysis',
-      isEnabledByDefault: false,
+      isEnabledByDefault: true,
       isExpired: false,
-      experimentEnabledVersion: defaultLanguageVersion,
-      experimentReleasedVersion: defaultLanguageVersion),
+      experimentEnabledVersion: const Version(3, 9),
+      experimentReleasedVersion: const Version(3, 9)),
 
   spreadCollections(
       name: 'spread-collections',
diff --git a/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/equality_operator.dart b/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/equality_operator.dart
index 30a2a57ad61..9c6c5910319 100644
--- a/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/equality_operator.dart
+++ b/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/equality_operator.dart
@@ -35,9 +35,9 @@ void nullableValue(int? x) {
 }
 
 void nonNullableValue(int x) {
-  if (x == null) {
+  if (x == null) /*unreachable*/{
     // Reachable since the value of x might come from legacy code
-    1;
+    /*stmt: unreachable*/1;
   } else {
     2;
   }
@@ -68,9 +68,9 @@ void potentiallyNullableTypeVar_nullableBound<T extends Object?>(T x) {
 }
 
 void nonNullableTypeVar<T extends Object>(T x) {
-  if (x == null) {
+  if (x == null) /*unreachable*/{
     // Reachable since the value of x might come from legacy code
-    1;
+    /*stmt: unreachable*/1;
   } else {
     2;
   }
diff --git a/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/if_null.dart b/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/if_null.dart
index fa2053df93d..6634d1b1915 100644
--- a/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/if_null.dart
+++ b/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/if_null.dart
@@ -12,18 +12,18 @@ void variable_if_null_assign_reachable(int? i) {
 
 void variable_if_null_unreachable(int i) {
   // Reachable since the value of i might come from legacy code
-  i ?? 0;
+  i ?? /*unreachable*/0;
 }
 
 void variable_if_null_assign_unreachable(int i) {
   // Reachable since the value of i might come from legacy code
-  i ??= 0;
+  /*cfe.update: unreachable*/i ??= /*unreachable*/0;
 }
 
 void variable_if_null_assign_unreachable_due_to_promotion(int? i) {
   if (i == null) return;
   // Reachable since the value of i might come from legacy code
-  i ??= 0;
+  /*cfe.update: unreachable*/i ??= /*unreachable*/0;
 }
 
 /*member: topLevelNullable:doesNotComplete*/
@@ -45,13 +45,13 @@ void top_level_if_null_assign_reachable() {
 void top_level_if_null_unreachable() {
   // Reachable since the value returned by topLevelNonNullGet might come from
   // legacy code
-  topLevelNonNullGet ?? 0;
+  topLevelNonNullGet ?? /*unreachable*/0;
 }
 
 void top_level_if_null_assign_unreachable() {
   // Reachable since the value returned by topLevelNonNullGet might come from
   // legacy code
-  topLevelNonNullGet ??= 0;
+  topLevelNonNullGet /*cfe.update: unreachable*/??= /*unreachable*/0;
 }
 
 class HasProperty<T> {
@@ -70,12 +70,12 @@ void property_if_null_assign_reachable(HasProperty<int?> x) {
 
 void property_if_null_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by prop might come from legacy code
-  x.prop ?? 0;
+  x.prop ?? /*unreachable*/0;
 }
 
 void property_if_null_assign_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by prop might come from legacy code
-  x.prop ??= 0;
+  x.prop ??= /*unreachable*/0;
 }
 
 void null_aware_property_if_null_reachable(HasProperty<int?>? x) {
@@ -94,7 +94,7 @@ void null_aware_property_if_null_not_shortened(HasProperty<int>? x) {
 
 void null_aware_property_if_null_assign_unreachable(HasProperty<int>? x) {
   // Reachable since the value returned by prop might come from legacy code.
-  x?.prop ??= 0;
+  x?.prop ??= /*unreachable*/0;
 }
 
 class SuperIntQuestionProperty extends HasProperty<int?> {
@@ -110,12 +110,12 @@ class SuperIntQuestionProperty extends HasProperty<int?> {
 class SuperIntProperty extends HasProperty<int> {
   void if_null_unreachable() {
     // Reachable since the value returned by prop might come from legacy code.
-    super.prop ?? 0;
+    super.prop ?? /*unreachable*/0;
   }
 
   void if_null_assign_unreachable() {
     // Reachable since the value returned by prop might come from legacy code.
-    super.prop ??= 0;
+    super.prop ??= /*unreachable*/0;
   }
 }
 
@@ -138,13 +138,13 @@ void extended_property_if_null_assign_reachable(HasProperty<int?> x) {
 void extended_property_if_null_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by extendedProp might come from legacy
   // code.
-  x.extendedProp ?? 0;
+  x.extendedProp ?? /*unreachable*/0;
 }
 
 void extended_property_if_null_assign_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by extendedProp might come from legacy
   // code.
-  x.extendedProp ??= 0;
+  x.extendedProp ??= /*unreachable*/0;
 }
 
 void null_aware_extended_property_if_null_reachable(HasProperty<int?>? x) {
@@ -166,7 +166,7 @@ void null_aware_extended_property_if_null_assign_unreachable(
     HasProperty<int>? x) {
   // Reachable since the value returned by extendedProp might come from legacy
   // code.
-  x?.extendedProp ??= 0;
+  x?.extendedProp ??= /*unreachable*/0;
 }
 
 void explicit_extended_property_if_null_reachable(HasProperty<int?> x) {
@@ -180,13 +180,13 @@ void explicit_extended_property_if_null_assign_reachable(HasProperty<int?> x) {
 void explicit_extended_property_if_null_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by extendedProp might come from legacy
   // code.
-  ExtensionProperty(x).extendedProp ?? 0;
+  ExtensionProperty(x).extendedProp ?? /*unreachable*/0;
 }
 
 void explicit_extended_property_if_null_assign_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by extendedProp might come from legacy
   // code.
-  ExtensionProperty(x).extendedProp ??= 0;
+  ExtensionProperty(x).extendedProp ??= /*unreachable*/0;
 }
 
 void null_aware_explicit_extended_property_if_null_reachable(
@@ -210,7 +210,7 @@ void null_aware_explicit_extended_property_if_null_assign_unreachable(
     HasProperty<int>? x) {
   // Reachable since the value returned by extendedProp might come from legacy
   // code.
-  ExtensionProperty(x)?.extendedProp ??= 0;
+  ExtensionProperty(x)?.extendedProp ??= /*unreachable*/0;
 }
 
 class Indexable<T> {
@@ -226,7 +226,7 @@ void index_if_null_reachable(Indexable<int?> x) {
 void index_if_null_unreachable(Indexable<int> x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  x[0] ?? 0;
+  x[0] ?? /*unreachable*/0;
 }
 
 void index_if_null_assign_reachable(Indexable<int?> x) {
@@ -236,7 +236,7 @@ void index_if_null_assign_reachable(Indexable<int?> x) {
 void index_if_null_assign_unreachable(Indexable<int> x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  x[0] ??= 0;
+  x[0] ??= /*unreachable*/0;
 }
 
 void null_aware_index_if_null_reachable(Indexable<int?>? x) {
@@ -256,7 +256,7 @@ void null_aware_index_if_null_assign_reachable(Indexable<int?>? x) {
 void null_aware_index_if_null_assign_unreachable(Indexable<int>? x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  x?[0] ??= 0;
+  x?[0] ??= /*unreachable*/0;
 }
 
 class SuperIntQuestionIndex extends Indexable<int?> {
@@ -273,13 +273,13 @@ class SuperIntIndex extends Indexable<int> {
   void if_null_unreachable() {
     // Reachable since the value returned by operator[] might come from legacy
     // code.
-    super[0] ?? 0;
+    super[0] ?? /*unreachable*/0;
   }
 
   void if_null_assign_unreachable() {
     // Reachable since the value returned by operator[] might come from legacy
     // code.
-    super[0] ??= 0;
+    super[0] ??= /*unreachable*/0;
   }
 }
 
@@ -302,13 +302,13 @@ void extended_index_if_null_assign_reachable(HasProperty<int?> x) {
 void extended_index_if_null_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  x[0] ?? 0;
+  x[0] ?? /*unreachable*/0;
 }
 
 void extended_index_if_null_assign_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  x[0] ??= 0;
+  x[0] ??= /*unreachable*/0;
 }
 
 void null_aware_extended_index_if_null_reachable(HasProperty<int?>? x) {
@@ -328,7 +328,7 @@ void null_aware_extended_index_if_null_not_shortened(HasProperty<int>? x) {
 void null_aware_extended_index_if_null_assign_unreachable(HasProperty<int>? x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  x?[0] ??= 0;
+  x?[0] ??= /*unreachable*/0;
 }
 
 void explicit_extended_index_if_null_reachable(HasProperty<int?> x) {
@@ -342,13 +342,13 @@ void explicit_extended_index_if_null_assign_reachable(HasProperty<int?> x) {
 void explicit_extended_index_if_null_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  ExtensionIndex(x)[0] ?? 0;
+  ExtensionIndex(x)[0] ?? /*unreachable*/0;
 }
 
 void explicit_extended_index_if_null_assign_unreachable(HasProperty<int> x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  ExtensionIndex(x)[0] ??= 0;
+  ExtensionIndex(x)[0] ??= /*unreachable*/0;
 }
 
 void null_aware_explicit_extended_index_if_null_reachable(
@@ -372,5 +372,5 @@ void null_aware_explicit_extended_index_if_null_assign_unreachable(
     HasProperty<int>? x) {
   // Reachable since the value returned by operator[] might come from legacy
   // code.
-  ExtensionIndex(x)?[0] ??= 0;
+  ExtensionIndex(x)?[0] ??= /*unreachable*/0;
 }
diff --git a/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/null_aware_access.dart b/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/null_aware_access.dart
index 43c8e2e1ee2..09c8c90d864 100644
--- a/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/null_aware_access.dart
+++ b/pkg/_fe_analyzer_shared/test/flow_analysis/reachability/data/null_aware_access.dart
@@ -7,10 +7,11 @@ void index_reachable(List<int>? f()) {
   0;
 }
 
+/*member: index_unreachable:doesNotComplete*/
 void index_unreachable(List<int> f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?[throw ''];
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void cascaded_index_reachable(List<int>? f()) {
@@ -18,10 +19,11 @@ void cascaded_index_reachable(List<int>? f()) {
   0;
 }
 
+/*member: cascaded_index_unreachable:doesNotComplete*/
 void cascaded_index_unreachable(List<int> f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?..[throw ''];
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void method_invocation_reachable(int? f()) {
@@ -29,10 +31,11 @@ void method_invocation_reachable(int? f()) {
   0;
 }
 
+/*member: method_invocation_unreachable:doesNotComplete*/
 void method_invocation_unreachable(int f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?.remainder(throw '');
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void cascaded_method_invocation_reachable(int? f()) {
@@ -40,10 +43,11 @@ void cascaded_method_invocation_reachable(int? f()) {
   0;
 }
 
+/*member: cascaded_method_invocation_unreachable:doesNotComplete*/
 void cascaded_method_invocation_unreachable(int f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?..remainder(throw '');
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void property_get_reachable(int? f()) {
@@ -51,10 +55,11 @@ void property_get_reachable(int? f()) {
   0;
 }
 
+/*member: property_get_unreachable:doesNotComplete*/
 void property_get_unreachable(int f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?.hashCode.remainder(throw '');
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void cascaded_property_get_reachable(int? f()) {
@@ -62,10 +67,11 @@ void cascaded_property_get_reachable(int? f()) {
   0;
 }
 
+/*member: cascaded_property_get_unreachable:doesNotComplete*/
 void cascaded_property_get_unreachable(int f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?..hashCode.remainder(throw '');
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void property_get_invocation_reachable(List<void Function(dynamic)>? f()) {
@@ -76,13 +82,14 @@ void property_get_invocation_reachable(List<void Function(dynamic)>? f()) {
   0;
 }
 
+/*member: property_get_invocation_unreachable:doesNotComplete*/
 void property_get_invocation_unreachable(List<void Function(dynamic)> f()) {
   // Reachable since the value returned by f() might come from legacy code
   // We need a special test case for this because it parses like a method
   // invocation but the analyzer rewrites it as a property access followed by a
   // function expression invocation.
   f()?.first(throw '');
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void cascaded_property_get_invocation_reachable(
@@ -94,6 +101,7 @@ void cascaded_property_get_invocation_reachable(
   0;
 }
 
+/*member: cascaded_property_get_invocation_unreachable:doesNotComplete*/
 void cascaded_property_get_invocation_unreachable(
     List<void Function(dynamic)> f()) {
   // Reachable since the value returned by f() might come from legacy code
@@ -101,7 +109,7 @@ void cascaded_property_get_invocation_unreachable(
   // invocation but the analyzer rewrites it as a property access followed by a
   // function expression invocation.
   f()?..first(throw '');
-  0;
+  /*stmt: unreachable*/0;
 }
 
 class C {
@@ -113,10 +121,11 @@ void property_set_reachable(C? f()) {
   0;
 }
 
+/*member: property_set_unreachable:doesNotComplete*/
 void property_set_unreachable(C f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?.field = throw '';
-  0;
+  /*stmt: unreachable*/0;
 }
 
 void cascaded_property_set_reachable(C? f()) {
@@ -124,8 +133,9 @@ void cascaded_property_set_reachable(C? f()) {
   0;
 }
 
+/*member: cascaded_property_set_unreachable:doesNotComplete*/
 void cascaded_property_set_unreachable(C f()) {
   // Reachable since the value returned by f() might come from legacy code
   f()?..field = throw '';
-  0;
+  /*stmt: unreachable*/0;
 }
diff --git a/pkg/analysis_server/test/src/services/correction/fix/remove_comparison_test.dart b/pkg/analysis_server/test/src/services/correction/fix/remove_comparison_test.dart
index 05c1d325c47..78d5b3282af 100644
--- a/pkg/analysis_server/test/src/services/correction/fix/remove_comparison_test.dart
+++ b/pkg/analysis_server/test/src/services/correction/fix/remove_comparison_test.dart
@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analysis_server/src/services/correction/fix.dart';
+import 'package:analyzer/diagnostic/diagnostic.dart';
+import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
 import 'package:linter/src/lint_names.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ -19,6 +21,9 @@ void main() {
   });
 }
 
+bool _ignoreDeadCode(Diagnostic diagnostic) =>
+    diagnostic.errorCode != WarningCode.DEAD_CODE;
+
 @reflectiveTest
 class RemoveComparisonTest extends FixProcessorTest {
   @override
@@ -168,7 +173,7 @@ void f(int x) {
     2,
   ];
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifElement_alwaysFalse_hasElse_withComments() async {
@@ -197,7 +202,7 @@ void f(int x) {
     2,
   ];
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifElement_alwaysFalse_noElse_insideList() async {
@@ -217,7 +222,7 @@ void f(int x) {
     2,
   ];
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void>
@@ -241,7 +246,7 @@ void f(int x) {
     2,
   ];
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifElement_alwaysFalse_noElse_insideSet() async {
@@ -261,7 +266,7 @@ Object f(int x) {
     2,
   };
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifElement_alwaysTrue() async {
@@ -304,7 +309,7 @@ void f(int x) {
     2,
   ];
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifElement_alwaysTrue_withComments() async {
@@ -351,7 +356,7 @@ void f(int x) {
   2;
   3;
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifStatement_alwaysFalse_hasElse_block_empty() async {
@@ -369,7 +374,7 @@ void f(int x) {
   0;
   2;
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifStatement_alwaysFalse_hasElse_statement() async {
@@ -389,7 +394,7 @@ void f(int x) {
   2;
   3;
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifStatement_alwaysFalse_noElse() async {
@@ -407,7 +412,7 @@ void f(int x) {
   0;
   2;
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifStatement_alwaysTrue_hasElse_block() async {
@@ -428,7 +433,7 @@ void f(int x) {
   1;
   3;
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_ifStatement_alwaysTrue_noElse() async {
@@ -745,7 +750,7 @@ class Person {
   }
 }
 ''');
-    await assertNoFix();
+    await assertNoFix(errorFilter: _ignoreDeadCode);
   }
 }
 
diff --git a/pkg/analysis_server/test/src/services/correction/fix/remove_if_null_operator_test.dart b/pkg/analysis_server/test/src/services/correction/fix/remove_if_null_operator_test.dart
index 2eab8e41940..703a37c8ed5 100644
--- a/pkg/analysis_server/test/src/services/correction/fix/remove_if_null_operator_test.dart
+++ b/pkg/analysis_server/test/src/services/correction/fix/remove_if_null_operator_test.dart
@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analysis_server/src/services/correction/fix.dart';
+import 'package:analyzer/diagnostic/diagnostic.dart';
+import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer_plugin/utilities/fixes/fixes.dart';
 import 'package:linter/src/lint_names.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
@@ -18,6 +20,9 @@ void main() {
   });
 }
 
+bool _ignoreDeadCode(Diagnostic diagnostic) =>
+    diagnostic.errorCode != WarningCode.DEAD_CODE;
+
 @reflectiveTest
 class DeadNullAwareAssignmentExpressionTest extends FixProcessorTest {
   @override
@@ -46,7 +51,7 @@ void f() {
 }
 
 C g() => C();
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_assignmentExpression_simpleIdentifier_field() async {
@@ -64,7 +69,7 @@ class C {
   void f(int b) {
   }
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_assignmentExpression_simpleIdentifier_parameter() async {
@@ -76,7 +81,7 @@ void f(int a, int b) {
     await assertHasFix('''
 void f(int a, int b) {
 }
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_immediateChild() async {
@@ -85,7 +90,7 @@ void f(int a, int b) => a ??= b;
 ''');
     await assertHasFix('''
 void f(int a, int b) => a;
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_nestedChild() async {
@@ -94,7 +99,7 @@ void f(int a, int b) => a ??= b * 2 + 1;
 ''');
     await assertHasFix('''
 void f(int a, int b) => a;
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_nestedChild_onRight() async {
@@ -103,7 +108,7 @@ void f(int a, int b, int c) => a = b ??= c;
 ''');
     await assertHasFix('''
 void f(int a, int b, int c) => a = b;
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 }
 
@@ -118,7 +123,7 @@ int f(int a, int b) => a ?? b;
 ''');
     await assertHasFix('''
 int f(int a, int b) => a;
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 
   Future<void> test_nestedChild() async {
@@ -127,7 +132,7 @@ int f(int a, int b) => a ?? b * 2 + 1;
 ''');
     await assertHasFix('''
 int f(int a, int b) => a;
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 }
 
@@ -194,6 +199,6 @@ var b = a ?? '';
     await assertHasFix('''
 var a = '';
 var b = a;
-''');
+''', errorFilter: _ignoreDeadCode);
   }
 }
diff --git a/pkg/analyzer/lib/src/dart/analysis/experiments.g.dart b/pkg/analyzer/lib/src/dart/analysis/experiments.g.dart
index 29226f9a31e..0dc17c3bbdf 100644
--- a/pkg/analyzer/lib/src/dart/analysis/experiments.g.dart
+++ b/pkg/analyzer/lib/src/dart/analysis/experiments.g.dart
@@ -528,7 +528,7 @@ class ExperimentalFeatures {
     documentation:
         'Assume sound null safety when computing type promotion, reachability, and definite assignment.',
     experimentalReleaseVersion: null,
-    releaseVersion: null,
+    releaseVersion: Version.parse('3.9.0'),
     channels: ["stable", "beta", "dev", "main"],
   );
 
@@ -717,7 +717,7 @@ class IsEnabledByDefault {
   static const bool set_literals = true;
 
   /// Default state of the experiment "sound-flow-analysis"
-  static const bool sound_flow_analysis = false;
+  static const bool sound_flow_analysis = true;
 
   /// Default state of the experiment "spread-collections"
   static const bool spread_collections = true;
diff --git a/pkg/analyzer/test/src/dart/analysis/analysis_context_collection_test.dart b/pkg/analyzer/test/src/dart/analysis/analysis_context_collection_test.dart
index 4c5312a1419..659469609d9 100644
--- a/pkg/analyzer/test/src/dart/analysis/analysis_context_collection_test.dart
+++ b/pkg/analyzer/test/src/dart/analysis/analysis_context_collection_test.dart
@@ -419,6 +419,7 @@ analysisOptions
       records
       sealed-class
       set-literals
+      sound-flow-analysis
       spread-collections
       super-parameters
       triple-shift
@@ -497,6 +498,7 @@ analysisOptions
       records
       sealed-class
       set-literals
+      sound-flow-analysis
       spread-collections
       super-parameters
       triple-shift
diff --git a/pkg/analyzer/test/src/dart/constant/evaluation_test.dart b/pkg/analyzer/test/src/dart/constant/evaluation_test.dart
index 84bdc4884aa..56b1d907e5f 100644
--- a/pkg/analyzer/test/src/dart/constant/evaluation_test.dart
+++ b/pkg/analyzer/test/src/dart/constant/evaluation_test.dart
@@ -1345,6 +1345,7 @@ const c = x ?? 1;
           23,
           1,
         ),
+        error(WarningCode.DEAD_CODE, 25, 4),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 28, 1),
       ],
     );
@@ -1357,6 +1358,7 @@ final x = 1;
 const c = 0 ?? x;
 ''',
       [
+        error(WarningCode.DEAD_CODE, 25, 4),
         error(
           CompileTimeErrorCode.CONST_INITIALIZED_WITH_NON_CONSTANT_VALUE,
           28,
diff --git a/pkg/analyzer/test/src/dart/resolution/ast_rewrite_test.dart b/pkg/analyzer/test/src/dart/resolution/ast_rewrite_test.dart
index b5ae43e473e..4530c672188 100644
--- a/pkg/analyzer/test/src/dart/resolution/ast_rewrite_test.dart
+++ b/pkg/analyzer/test/src/dart/resolution/ast_rewrite_test.dart
@@ -185,7 +185,10 @@ abstract class C extends A {
 
 void Function() f(A a, bool b, C c, dynamic d) => b ? d : c ?? a;
 ''',
-      [error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 130, 1)],
+      [
+        error(WarningCode.DEAD_CODE, 127, 4),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 130, 1),
+      ],
     );
     // `c` is on the LHS of an if-null expression, so implicit call tearoff
     // logic should not apply to it.
diff --git a/pkg/analyzer/test/src/diagnostics/constant_pattern_never_matches_value_type_test.dart b/pkg/analyzer/test/src/diagnostics/constant_pattern_never_matches_value_type_test.dart
index 872ee19094f..4f2720cf0ab 100644
--- a/pkg/analyzer/test/src/diagnostics/constant_pattern_never_matches_value_type_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/constant_pattern_never_matches_value_type_test.dart
@@ -398,7 +398,10 @@ void f(void Function() x) {
   if (x case (null)) {}
 }
 ''',
-      [error(WarningCode.CONSTANT_PATTERN_NEVER_MATCHES_VALUE_TYPE, 42, 4)],
+      [
+        error(WarningCode.CONSTANT_PATTERN_NEVER_MATCHES_VALUE_TYPE, 42, 4),
+        error(WarningCode.DEAD_CODE, 49, 2),
+      ],
     );
   }
 
@@ -417,7 +420,10 @@ void f(int x) {
   if (x case (null)) {}
 }
 ''',
-      [error(WarningCode.CONSTANT_PATTERN_NEVER_MATCHES_VALUE_TYPE, 30, 4)],
+      [
+        error(WarningCode.CONSTANT_PATTERN_NEVER_MATCHES_VALUE_TYPE, 30, 4),
+        error(WarningCode.DEAD_CODE, 37, 2),
+      ],
     );
   }
 
@@ -436,7 +442,10 @@ void f<T extends Object>(T x) {
   if (x case null) {}
 }
 ''',
-      [error(WarningCode.CONSTANT_PATTERN_NEVER_MATCHES_VALUE_TYPE, 45, 4)],
+      [
+        error(WarningCode.CONSTANT_PATTERN_NEVER_MATCHES_VALUE_TYPE, 45, 4),
+        error(WarningCode.DEAD_CODE, 51, 2),
+      ],
     );
   }
 
diff --git a/pkg/analyzer/test/src/diagnostics/dead_null_aware_expression_test.dart b/pkg/analyzer/test/src/diagnostics/dead_null_aware_expression_test.dart
index 0bd257f1fe4..43d71b88df4 100644
--- a/pkg/analyzer/test/src/diagnostics/dead_null_aware_expression_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/dead_null_aware_expression_test.dart
@@ -35,7 +35,10 @@ f(int x) {
   x ??= 0;
 }
 ''',
-      [error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 19, 1)],
+      [
+        error(WarningCode.DEAD_CODE, 19, 2),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 19, 1),
+      ],
     );
   }
 
@@ -54,7 +57,10 @@ f(int x) {
   x ?? 0;
 }
 ''',
-      [error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 18, 1)],
+      [
+        error(WarningCode.DEAD_CODE, 15, 4),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 18, 1),
+      ],
     );
   }
 
diff --git a/pkg/analyzer/test/src/diagnostics/not_assigned_potentially_non_nullable_local_variable_test.dart b/pkg/analyzer/test/src/diagnostics/not_assigned_potentially_non_nullable_local_variable_test.dart
index f74f9f6dff2..c210db2aeb4 100644
--- a/pkg/analyzer/test/src/diagnostics/not_assigned_potentially_non_nullable_local_variable_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/not_assigned_potentially_non_nullable_local_variable_test.dart
@@ -89,6 +89,7 @@ void f() {
 ''',
       [
         _notAssignedError(22, 1),
+        error(WarningCode.DEAD_CODE, 28, 2),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 28, 1),
       ],
     );
@@ -104,6 +105,7 @@ void f() {
 ''',
       [
         _notAssignedError(22, 1),
+        error(WarningCode.DEAD_CODE, 28, 2),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 28, 1),
         _notAssignedError(28, 1),
       ],
@@ -129,7 +131,10 @@ void f() {
   v;
 }
 ''',
-      [error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 33, 1)],
+      [
+        error(WarningCode.DEAD_CODE, 30, 4),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 33, 1),
+      ],
     );
   }
 
@@ -143,6 +148,7 @@ void f(int a) {
 }
 ''',
       [
+        error(WarningCode.DEAD_CODE, 29, 10),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 32, 7),
         _notAssignedError(43, 1),
       ],
diff --git a/pkg/analyzer/test/src/diagnostics/pattern_never_matches_value_type_test.dart b/pkg/analyzer/test/src/diagnostics/pattern_never_matches_value_type_test.dart
index 02c1a7d33f3..5dcb79b5a9a 100644
--- a/pkg/analyzer/test/src/diagnostics/pattern_never_matches_value_type_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/pattern_never_matches_value_type_test.dart
@@ -958,7 +958,10 @@ void f(Null x) {
 
 class A {}
 ''',
-      [error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 30, 1)],
+      [
+        error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 30, 1),
+        error(WarningCode.DEAD_CODE, 35, 2),
+      ],
     );
   }
 
@@ -977,7 +980,10 @@ void f(Null x) {
   if (x case Object _) {}
 }
 ''',
-      [error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 30, 6)],
+      [
+        error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 30, 6),
+        error(WarningCode.DEAD_CODE, 40, 2),
+      ],
     );
   }
 
@@ -1277,7 +1283,10 @@ void f(A x) {
 
 class A {}
 ''',
-      [error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 27, 4)],
+      [
+        error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 27, 4),
+        error(WarningCode.DEAD_CODE, 35, 2),
+      ],
     );
   }
 
@@ -1288,7 +1297,10 @@ void f(void Function() x) {
   if (x case Null _) {}
 }
 ''',
-      [error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 41, 4)],
+      [
+        error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 41, 4),
+        error(WarningCode.DEAD_CODE, 49, 2),
+      ],
     );
   }
 
@@ -1299,7 +1311,10 @@ void f(Object x) {
   if (x case Null _) {}
 }
 ''',
-      [error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 32, 4)],
+      [
+        error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 32, 4),
+        error(WarningCode.DEAD_CODE, 40, 2),
+      ],
     );
   }
 
@@ -1310,7 +1325,10 @@ void f<T extends num>(T x) {
   if (x case Null _) {}
 }
 ''',
-      [error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 42, 4)],
+      [
+        error(WarningCode.PATTERN_NEVER_MATCHES_VALUE_TYPE, 42, 4),
+        error(WarningCode.DEAD_CODE, 50, 2),
+      ],
     );
   }
 
diff --git a/pkg/analyzer/test/src/diagnostics/relational_pattern_operand_type_not_assignable_test.dart b/pkg/analyzer/test/src/diagnostics/relational_pattern_operand_type_not_assignable_test.dart
index 9667f6a4980..ccdfe4d4477 100644
--- a/pkg/analyzer/test/src/diagnostics/relational_pattern_operand_type_not_assignable_test.dart
+++ b/pkg/analyzer/test/src/diagnostics/relational_pattern_operand_type_not_assignable_test.dart
@@ -122,7 +122,8 @@ void f(A? x) {
   }
 
   test_eqEq_operandNull() async {
-    await assertNoErrorsInCode(r'''
+    await assertErrorsInCode(
+      r'''
 class A {}
 
 void f(A x) {
@@ -131,7 +132,9 @@ void f(A x) {
       break;
   }
 }
-''');
+''',
+      [error(WarningCode.DEAD_CODE, 65, 6)],
+    );
   }
 
   test_eqEq_operandNullable() async {
diff --git a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
index 2d958e0a20c..a5437ab7f4a 100644
--- a/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
+++ b/pkg/analyzer/test/src/task/strong/inferred_type_test.dart
@@ -5381,6 +5381,7 @@ main() {
           34,
           1,
         ),
+        error(WarningCode.DEAD_CODE, 36, 5),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 39, 2),
         error(WarningCode.UNUSED_LOCAL_VARIABLE, 55, 1),
       ],
@@ -5404,6 +5405,7 @@ main() {
           40,
           1,
         ),
+        error(WarningCode.DEAD_CODE, 42, 5),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 45, 2),
       ],
     );
diff --git a/pkg/analyzer/test/verify_diagnostics_test.dart b/pkg/analyzer/test/verify_diagnostics_test.dart
index bfc218d609c..58ab49a49a6 100644
--- a/pkg/analyzer/test/verify_diagnostics_test.dart
+++ b/pkg/analyzer/test/verify_diagnostics_test.dart
@@ -176,6 +176,9 @@ class DocumentationValidator {
     'PubspecWarningCode.WORKSPACE_VALUE_NOT_STRING',
     'PubspecWarningCode.WORKSPACE_VALUE_NOT_SUBDIRECTORY',
 
+    // Produces two diagnostics out of necessity.
+    'StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION',
+
     // Reports CompileTimeErrorCode.FINAL_CLASS_EXTENDED_OUTSIDE_OF_LIBRARY
     'WarningCode.DEPRECATED_EXTENDS_FUNCTION',
     // Produces more than one error range by design.
@@ -183,6 +186,8 @@ class DocumentationValidator {
     'WarningCode.TEXT_DIRECTION_CODE_POINT_IN_COMMENT',
     // Produces more than one error range by design.
     'WarningCode.TEXT_DIRECTION_CODE_POINT_IN_LITERAL',
+    // Produces two diagnostics out of necessity.
+    'WarningCode.UNNECESSARY_NULL_COMPARISON_NEVER_NULL_FALSE',
   ];
 
   /// The buffer to which validation errors are written.
diff --git a/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart b/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart
index 19d097f17ab..4b6ab154b56 100644
--- a/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart
+++ b/pkg/front_end/lib/src/api_prototype/experimental_flags_generated.dart
@@ -301,11 +301,11 @@ class ExperimentalFlag {
 
   static const ExperimentalFlag soundFlowAnalysis = const ExperimentalFlag(
       name: 'sound-flow-analysis',
-      isEnabledByDefault: false,
+      isEnabledByDefault: true,
       isExpired: false,
-      enabledVersion: defaultLanguageVersion,
-      experimentEnabledVersion: defaultLanguageVersion,
-      experimentReleasedVersion: defaultLanguageVersion);
+      enabledVersion: const Version(3, 9),
+      experimentEnabledVersion: const Version(3, 9),
+      experimentReleasedVersion: const Version(3, 9));
 
   static const ExperimentalFlag spreadCollections = const ExperimentalFlag(
       name: 'spread-collections',
diff --git a/pkg/linter/test/rules/avoid_null_checks_in_equality_operators_test.dart b/pkg/linter/test/rules/avoid_null_checks_in_equality_operators_test.dart
index 1a2f4cf363a..51e2c40036a 100644
--- a/pkg/linter/test/rules/avoid_null_checks_in_equality_operators_test.dart
+++ b/pkg/linter/test/rules/avoid_null_checks_in_equality_operators_test.dart
@@ -110,6 +110,7 @@ class C {
       [
         error(WarningCode.NON_NULLABLE_EQUALS_PARAMETER, 62, 2),
         lint(126, 14),
+        error(WarningCode.DEAD_CODE, 132, 8),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 135, 5),
       ],
     );
diff --git a/pkg/linter/test/rules/invalid_runtime_check_with_js_interop_types_test.dart b/pkg/linter/test/rules/invalid_runtime_check_with_js_interop_types_test.dart
index 513096069fa..f1350117a5b 100644
--- a/pkg/linter/test/rules/invalid_runtime_check_with_js_interop_types_test.dart
+++ b/pkg/linter/test/rules/invalid_runtime_check_with_js_interop_types_test.dart
@@ -442,7 +442,10 @@ class InvalidRuntimeCheckWithJSInteropTypesTest extends LintRuleTest {
       null as JSArray?;
     }
     ''',
-      [error(WarningCode.CAST_FROM_NULL_ALWAYS_FAILS, 54, 13)],
+      [
+        error(WarningCode.CAST_FROM_NULL_ALWAYS_FAILS, 54, 13),
+        error(WarningCode.DEAD_CODE, 75, 17),
+      ],
     );
   }
 
diff --git a/pkg/linter/test/rules/parameter_assignments_test.dart b/pkg/linter/test/rules/parameter_assignments_test.dart
index 919e813c60e..4d41894e7b3 100644
--- a/pkg/linter/test/rules/parameter_assignments_test.dart
+++ b/pkg/linter/test/rules/parameter_assignments_test.dart
@@ -151,6 +151,7 @@ void f([int? optional]) {
       [
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 59, 2),
         lint(46, 15),
+        error(WarningCode.DEAD_CODE, 59, 3),
       ],
     );
   }
@@ -305,7 +306,7 @@ void f([int p = 42]) {
   p ??= 8;
 }
 ''',
-      [lint(65, 7)],
+      [lint(65, 7), error(WarningCode.DEAD_CODE, 71, 2)],
     );
   }
 
@@ -317,7 +318,7 @@ void f({int p = 42}) {
   p ??= 8;
 }
 ''',
-      [lint(65, 7)],
+      [lint(65, 7), error(WarningCode.DEAD_CODE, 71, 2)],
     );
   }
 
@@ -346,7 +347,7 @@ void f([int? p]) {
   p ??= 16;
 }
 ''',
-      [lint(72, 8)],
+      [lint(72, 8), error(WarningCode.DEAD_CODE, 78, 3)],
     );
   }
 
diff --git a/pkg/linter/test/rules/unnecessary_null_in_if_null_operators_test.dart b/pkg/linter/test/rules/unnecessary_null_in_if_null_operators_test.dart
index 846a4ecf943..aec4ab70e08 100644
--- a/pkg/linter/test/rules/unnecessary_null_in_if_null_operators_test.dart
+++ b/pkg/linter/test/rules/unnecessary_null_in_if_null_operators_test.dart
@@ -24,7 +24,10 @@ void f() {
   var x = 1 ?? 1;
 }
 ''',
-      [error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 26, 1)],
+      [
+        error(WarningCode.DEAD_CODE, 23, 4),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 26, 1),
+      ],
     );
   }
 
@@ -46,7 +49,11 @@ void f() {
   var x = 1 ?? null;
 }
 ''',
-      [lint(26, 4), error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 26, 4)],
+      [
+        error(WarningCode.DEAD_CODE, 23, 7),
+        lint(26, 4),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 26, 4),
+      ],
     );
   }
 
@@ -61,6 +68,7 @@ class C {
 }
 ''',
       [
+        error(WarningCode.DEAD_CODE, 32, 7),
         lint(35, 4),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 35, 4),
         lint(53, 4),
@@ -79,6 +87,7 @@ class C {
 ''',
       [
         // No lint.
+        error(WarningCode.DEAD_CODE, 32, 4),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 35, 1),
       ],
     );
@@ -91,6 +100,7 @@ var x = 1 ?? null;
 var y = null ?? 1;
 ''',
       [
+        error(WarningCode.DEAD_CODE, 10, 7),
         lint(13, 4),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 13, 4),
         lint(27, 4),
@@ -105,6 +115,7 @@ var x = 1 ?? 1;
 ''',
       [
         // No lint.
+        error(WarningCode.DEAD_CODE, 10, 4),
         error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 13, 1),
       ],
     );
@@ -115,7 +126,10 @@ var x = 1 ?? 1;
       r'''
 var x = 1 ?? 1;
 ''',
-      [error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 13, 1)],
+      [
+        error(WarningCode.DEAD_CODE, 10, 4),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 13, 1),
+      ],
     );
   }
 
@@ -133,7 +147,11 @@ var x = null ?? 1;
       r'''
 var x = 1 ?? null;
 ''',
-      [lint(13, 4), error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 13, 4)],
+      [
+        error(WarningCode.DEAD_CODE, 10, 7),
+        lint(13, 4),
+        error(StaticWarningCode.DEAD_NULL_AWARE_EXPRESSION, 13, 4),
+      ],
     );
   }
 }
diff --git a/pkg/linter/test/rules/unrelated_type_equality_checks_test.dart b/pkg/linter/test/rules/unrelated_type_equality_checks_test.dart
index e50ed136b09..9ba287c5186 100644
--- a/pkg/linter/test/rules/unrelated_type_equality_checks_test.dart
+++ b/pkg/linter/test/rules/unrelated_type_equality_checks_test.dart
@@ -251,6 +251,7 @@ void f() {
       [
         // No lint.
         error(WarningCode.UNNECESSARY_NULL_COMPARISON_NEVER_NULL_FALSE, 23, 7),
+        error(WarningCode.DEAD_CODE, 32, 2),
       ],
     );
   }
diff --git a/runtime/vm/experimental_features.cc b/runtime/vm/experimental_features.cc
index 09a78fc518d..6486e6c1933 100644
--- a/runtime/vm/experimental_features.cc
+++ b/runtime/vm/experimental_features.cc
@@ -18,7 +18,7 @@ bool GetExperimentalFeatureDefault(ExperimentalFeature feature) {
   constexpr bool kFeatureValues[] = {
       true, true, true, true, true, true, true, true, true,
       true, true, true, true, true, true, true, true, true,
-      true, true, true, true, true, true, true, true,
+      true, true, true, true, true, true, true, true, true,
   };
   ASSERT(static_cast<size_t>(feature) < ARRAY_SIZE(kFeatureValues));
   return kFeatureValues[static_cast<int>(feature)];
@@ -26,6 +26,7 @@ bool GetExperimentalFeatureDefault(ExperimentalFeature feature) {
 
 const char* GetExperimentalFeatureName(ExperimentalFeature feature) {
   constexpr const char* kFeatureNames[] = {
+      "sound-flow-analysis",
       "null-aware-elements",
       "inference-using-bounds",
       "wildcard-variables",
diff --git a/runtime/vm/experimental_features.h b/runtime/vm/experimental_features.h
index 791314dec56..fa8b6ba2f2f 100644
--- a/runtime/vm/experimental_features.h
+++ b/runtime/vm/experimental_features.h
@@ -12,6 +12,7 @@
 namespace dart {
 
 enum class ExperimentalFeature {
+  sound_flow_analysis,
   null_aware_elements,
   inference_using_bounds,
   wildcard_variables,
diff --git a/tests/language/nnbd/flow_analysis/unreachable_via_this_test.dart b/tests/language/nnbd/flow_analysis/unreachable_via_this_test.dart
index 0a3ee26c7b4..7e7face0b63 100644
--- a/tests/language/nnbd/flow_analysis/unreachable_via_this_test.dart
+++ b/tests/language/nnbd/flow_analysis/unreachable_via_this_test.dart
@@ -2,10 +2,8 @@
 // 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.
 
-/// This test verifies that `is` and `==` tests performed on `this` do not lead
-/// to code being considered unreachable.  (In principle, we could soundly mark
-/// some such code as unreachable, but we have decided not to do so at this
-/// time).
+/// This test verifies that `is` and `==` tests performed on `this` lead to code
+/// being considered unreachable. (Prior to Dart 3.9, they didn't do so.)
 
 import 'package:expect/static_type_helper.dart';
 
@@ -19,9 +17,9 @@ class C {
     } else {
       y = null;
     }
-    // Since the assignments to x and y were both reachable, they should have
-    // static type `int?` now.
-    x.expectStaticType<Exactly<int?>>();
+    // Since the assignment to y was reachable, it should have static type
+    // `int?` now.
+    x.expectStaticType<Exactly<int>>();
     y.expectStaticType<Exactly<int?>>();
   }
 
@@ -36,10 +34,10 @@ class C {
         y = null;
       }
     }
-    // Since the assignments to x and y were both reachable, they should have
-    // static type `int?` now.
-    x.expectStaticType<Exactly<int?>>();
-    y.expectStaticType<Exactly<int?>>();
+    // Since the assignments to x and y were both unreachable, they should still
+    // have static type `int`.
+    x.expectStaticType<Exactly<int>>();
+    y.expectStaticType<Exactly<int>>();
   }
 
   void isSimple(int? x, int? y) {
@@ -68,10 +66,10 @@ class C {
         y = null;
       }
     }
-    // Since the assignment to y was reachable, it should have static type
-    // `int?` now.  But x should still have static type `int`.
+    // Since the assignments to x an y were both unreachable, they should still
+    // have static type `int`.
     x.expectStaticType<Exactly<int>>();
-    y.expectStaticType<Exactly<int?>>();
+    y.expectStaticType<Exactly<int>>();
   }
 }
 
@@ -87,9 +85,9 @@ extension on D {
     } else {
       y = null;
     }
-    // Since the assignments to x and y were both reachable, they should have
-    // static type `int?` now.
-    x.expectStaticType<Exactly<int?>>();
+    // Since the assignment to y was reachable, it should have static type
+    // `int?` now.
+    x.expectStaticType<Exactly<int>>();
     y.expectStaticType<Exactly<int?>>();
   }
 
@@ -104,10 +102,10 @@ extension on D {
         y = null;
       }
     }
-    // Since the assignments to x and y were both reachable, they should have
-    // static type `int?` now.
-    x.expectStaticType<Exactly<int?>>();
-    y.expectStaticType<Exactly<int?>>();
+    // Since the assignments to x and y were both unreachable, they should still
+    // have static type `int`.
+    x.expectStaticType<Exactly<int>>();
+    y.expectStaticType<Exactly<int>>();
   }
 
   void isSimple(int? x, int? y) {
@@ -136,10 +134,10 @@ extension on D {
         y = null;
       }
     }
-    // Since the assignment to y was reachable, it should have static type
-    // `int?` now.  But x should still have static type `int`.
+    // Since the assignments to x and y were both unreachable, they should still
+    // have static type `int`.
     x.expectStaticType<Exactly<int>>();
-    y.expectStaticType<Exactly<int?>>();
+    y.expectStaticType<Exactly<int>>();
   }
 }
 
diff --git a/tests/language/patterns/flow_analysis/constant_pattern_error_test.dart b/tests/language/patterns/flow_analysis/constant_pattern_error_test.dart
index 7f33841d9c7..32869c87997 100644
--- a/tests/language/patterns/flow_analysis/constant_pattern_error_test.dart
+++ b/tests/language/patterns/flow_analysis/constant_pattern_error_test.dart
@@ -72,12 +72,10 @@ test() {
     reachability1.expectStaticType<Exactly<int?>>();
   }
   {
-    // `case null` can even match non-nullable types.
+    // `case null` cannot match non-nullable types.
 
-    // Note that in most cases, flow analysis assumes soundness when analyzing
-    // patterns. This is an exception: `case null` is assumed to be a possible
-    // match even for a non-nullable scrutinee; this makes `case null` behave
-    // similarly to an `if (x == null)` test.
+    // As of Dart 3.9, flow analysis assumes soundness when analyzing all
+    // patterns, so `case null` cannot match a non-nullable type.
     int? reachability0 = 0;
     int? reachability1 = 0;
     if (expr<int>() case null) {
@@ -85,7 +83,7 @@ test() {
     } else {
       reachability1 = null;
     }
-    reachability0.expectStaticType<Exactly<int?>>();
+    reachability0.expectStaticType<Exactly<int>>();
     reachability1.expectStaticType<Exactly<int?>>();
   }
   {
diff --git a/tests/language/patterns/flow_analysis/relational_pattern_error_test.dart b/tests/language/patterns/flow_analysis/relational_pattern_error_test.dart
index 3bc9cb5b1c1..87625082e91 100644
--- a/tests/language/patterns/flow_analysis/relational_pattern_error_test.dart
+++ b/tests/language/patterns/flow_analysis/relational_pattern_error_test.dart
@@ -86,12 +86,10 @@ test() {
     reachability1.expectStaticType<Exactly<int?>>();
   }
   {
-    // An `== null` pattern can even match non-nullable types.
+    // An `== null` pattern cannot match non-nullable types.
 
-    // Note that in most cases, flow analysis assumes soundness when analyzing
-    // patterns. This is an exception: `case == null` is assumed to be a
-    // possible match even for a non-nullable scrutinee; this makes `case ==
-    // null` behave similarly to an `if (x == null)` test.
+    // As of Dart 3.9, flow analysis assumes soundness when analyzing all
+    // patterns, so `== null` cannot match a non-nullable type.
     int? reachability0 = 0;
     int? reachability1 = 0;
     if (expr<int>() case == null) {
@@ -99,7 +97,7 @@ test() {
     } else {
       reachability1 = null;
     }
-    reachability0.expectStaticType<Exactly<int?>>();
+    reachability0.expectStaticType<Exactly<int>>();
     reachability1.expectStaticType<Exactly<int?>>();
   }
   {
@@ -178,13 +176,11 @@ test() {
     }
   }
   {
-    // A `!= null` pattern may or may not match, even if the matched value type
-    // if non-nullable.
+    // A `!= null` pattern always matches, if the matched value type is
+    // non-nullable.
 
-    // Note that in most cases, flow analysis assumes soundness when analyzing
-    // patterns. This is an exception: `case != null` is assumed to be a
-    // possible mismatch even for a non-nullable scrutinee; this makes `case !=
-    // null` behave similarly to an `if (x != null)` test.
+    // As of Dart 3.9, flow analysis assumes soundness when analyzing all
+    // patterns, so `!= null` always matches a non-nullable type.
     int? reachability0 = 0;
     int? reachability1 = 0;
     if (expr<int>() case != null) {
@@ -193,7 +189,7 @@ test() {
       reachability1 = null;
     }
     reachability0.expectStaticType<Exactly<int?>>();
-    reachability1.expectStaticType<Exactly<int?>>();
+    reachability1.expectStaticType<Exactly<int>>();
   }
   {
     // If a relational pattern using `!=` appears inside a record pattern, the
diff --git a/tests/language/patterns/switch_trivial_exhaustiveness_error_test.dart b/tests/language/patterns/switch_trivial_exhaustiveness_error_test.dart
index 624f63b4134..0a362fa771d 100644
--- a/tests/language/patterns/switch_trivial_exhaustiveness_error_test.dart
+++ b/tests/language/patterns/switch_trivial_exhaustiveness_error_test.dart
@@ -308,8 +308,8 @@ void testLogicalOrNeitherMatches(Object x) {
 }
 
 void testNullCheckAlwaysMatches(Object x) {
-  // TODO(paulberry): should be trivially exhaustive because the matched value
-  // type is non-nullable and the subpattern always matches
+  // Trivially exhaustive because the matched value type is non-nullable and the
+  // subpattern always matches
   bool? y;
   switch (x) {
     case _?:
@@ -317,7 +317,7 @@ void testNullCheckAlwaysMatches(Object x) {
       // [analyzer] STATIC_WARNING.UNNECESSARY_NULL_CHECK_PATTERN
       y = true;
   }
-  y.expectStaticType<Exactly<bool?>>();
+  y.expectStaticType<Exactly<bool>>();
 }
 
 void testNullCheckNullableMatchedValueType(Object? x) {
@@ -635,13 +635,13 @@ void testWildcardUntyped(Object x) {
 }
 
 void testRelationalNotEqualsNullWithNonNullableScrutinee(Object x) {
-  // TODO(paulberry): this should be trivially exhaustive
+  // Trivially exhaustive because the matched value type is non-nullable
   bool? y;
   switch (x) {
     case != null:
       y = true;
   }
-  y.expectStaticType<Exactly<bool?>>();
+  y.expectStaticType<Exactly<bool>>();
 }
 
 void testRelationalNotEqualsNullWithNullableScrutinee(Object? x) {
diff --git a/tools/experimental_features.yaml b/tools/experimental_features.yaml
index 07d1fe9285e..7cb05c9d6db 100644
--- a/tools/experimental_features.yaml
+++ b/tools/experimental_features.yaml
@@ -151,10 +151,6 @@ features:
   getter-setter-error:
     help: "Stop reporting errors about mismatching types in a getter/setter pair."
 
-  sound-flow-analysis:
-    # See https://github.com/dart-lang/language/issues/3100
-    help: "Assume sound null safety when computing type promotion, reachability, and definite assignment."
-
 # Experiment flag only used for testing.
   test-experiment:
     help: >-
@@ -168,6 +164,18 @@ features:
 # Shipped flags should be marked retired the following stable release.
 #
 
+  sound-flow-analysis:
+    enabledIn: '3.9.0'
+    validation: |
+      main() {
+        var x = 0;
+        String y;
+        if (x != null) y = 'feature enabled';
+        print(y);
+      }
+    # See https://github.com/dart-lang/language/issues/3100
+    help: "Assume sound null safety when computing type promotion, reachability, and definite assignment."
+
   null-aware-elements:
     enabledIn: '3.8.0'
     validation: |