diff --git a/pkg/front_end/lib/src/kernel/constant_evaluator.dart b/pkg/front_end/lib/src/kernel/constant_evaluator.dart
index 1f1aa25a040..7239370f13e 100644
--- a/pkg/front_end/lib/src/kernel/constant_evaluator.dart
+++ b/pkg/front_end/lib/src/kernel/constant_evaluator.dart
@@ -3380,19 +3380,23 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
     if (condition is AbortConstant) return condition;
 
     if (shouldBeUnevaluated) {
-      // Coverage-ignore-block(suite): Not run.
-      Expression? message = null;
-      if (statement.message != null) {
-        enterLazy();
-        Constant constant = _evaluateSubexpression(statement.message!);
-        if (constant is AbortConstant) return constant;
-        message = _wrap(constant);
-        leaveLazy();
+      if (instanceBuilder != null) {
+        Expression? message = null;
+        if (statement.message != null) {
+          enterLazy();
+          Constant constant = _evaluateSubexpression(statement.message!);
+          if (constant is AbortConstant) return constant;
+          message = _wrap(constant);
+          leaveLazy();
+        }
+        instanceBuilder!.asserts.add(new AssertStatement(_wrap(condition),
+            message: message,
+            conditionStartOffset: statement.conditionStartOffset,
+            conditionEndOffset: statement.conditionEndOffset));
+      } else {
+        assert(inExtensionTypeConstConstructor);
+        return null;
       }
-      instanceBuilder!.asserts.add(new AssertStatement(_wrap(condition),
-          message: message,
-          conditionStartOffset: statement.conditionStartOffset,
-          conditionEndOffset: statement.conditionEndOffset));
     } else if (condition is BoolConstant) {
       if (!condition.value) {
         if (statement.message == null) {
@@ -4543,6 +4547,13 @@ class ConstantEvaluator implements ExpressionVisitor<Constant> {
         Constant result = _handleFunctionInvocation(
             node.target.function, typeArguments, positional, named);
         inExtensionTypeConstConstructor = oldInExtensionTypeConstructor;
+        if (shouldBeUnevaluated) {
+          return unevaluated(
+              node,
+              new StaticInvocation(target,
+                  unevaluatedArguments(positional, named, arguments.types),
+                  isConst: node.isConst));
+        }
         return result;
       } else {
         return createEvaluationErrorConstant(
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart b/pkg/front_end/testcases/dart2js/issue56803.dart
new file mode 100644
index 00000000000..8100aeff21e
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart
@@ -0,0 +1,14 @@
+// Copyright (c) 2024, 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.
+
+const String? maybe = bool.fromEnvironment("not there") ? "string" : null;
+extension type const Ext._(String _) implements String {
+  const Ext()
+      : assert(maybe != null, "Must not be null"),
+        _ = "OK";
+}
+void main() {
+  var c = const Ext();
+  print(c);
+}
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.strong.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.expect
new file mode 100644
index 00000000000..b8303d9949b
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.expect
@@ -0,0 +1,38 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type Ext(core::String _) implements core::String {
+  abstract extension-type-member representation-field get _() → core::String;
+  constructor _ = self::Ext|constructor#_;
+  constructor tearoff _ = self::Ext|constructor#_#_#tearOff;
+  constructor • = self::Ext|constructor#;
+  constructor tearoff • = self::Ext|constructor#_#new#tearOff;
+}
+static const field core::String? maybe = #C5;
+static extension-type-member method Ext|constructor#_(core::String _) → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = _;
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#_#tearOff(core::String _) → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#_(_);
+static extension-type-member method Ext|constructor#() → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = "OK";
+  assert(!(#C5 == null), "Must not be null");
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#new#tearOff() → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#();
+static method main() → void {
+  self::Ext /* erasure=core::String */ c = #C6;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = eval /* from org-dartlang-testcase:///issue56803.dart */ const self::Ext|constructor#()
+}
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.strong.modular.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.modular.expect
new file mode 100644
index 00000000000..b8303d9949b
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.modular.expect
@@ -0,0 +1,38 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type Ext(core::String _) implements core::String {
+  abstract extension-type-member representation-field get _() → core::String;
+  constructor _ = self::Ext|constructor#_;
+  constructor tearoff _ = self::Ext|constructor#_#_#tearOff;
+  constructor • = self::Ext|constructor#;
+  constructor tearoff • = self::Ext|constructor#_#new#tearOff;
+}
+static const field core::String? maybe = #C5;
+static extension-type-member method Ext|constructor#_(core::String _) → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = _;
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#_#tearOff(core::String _) → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#_(_);
+static extension-type-member method Ext|constructor#() → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = "OK";
+  assert(!(#C5 == null), "Must not be null");
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#new#tearOff() → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#();
+static method main() → void {
+  self::Ext /* erasure=core::String */ c = #C6;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = eval /* from org-dartlang-testcase:///issue56803.dart */ const self::Ext|constructor#()
+}
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.strong.outline.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.outline.expect
new file mode 100644
index 00000000000..66031b47ffc
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.outline.expect
@@ -0,0 +1,36 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type Ext(core::String _) implements core::String {
+  abstract extension-type-member representation-field get _() → core::String;
+  constructor _ = self::Ext|constructor#_;
+  constructor tearoff _ = self::Ext|constructor#_#_#tearOff;
+  constructor • = self::Ext|constructor#;
+  constructor tearoff • = self::Ext|constructor#_#new#tearOff;
+}
+static const field core::String? maybe = const core::bool::fromEnvironment("not there") ?{core::String?} "string" : null;
+static extension-type-member method Ext|constructor#_(core::String _) → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = _;
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#_#tearOff(core::String _) → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#_(_);
+static extension-type-member method Ext|constructor#() → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = "OK";
+  assert(!(self::maybe == null), "Must not be null");
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#new#tearOff() → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#();
+static method main() → void
+  ;
+
+
+Extra constant evaluation status:
+Evaluated with empty environment: Not @ org-dartlang-testcase:///issue56803.dart:8:22 -> BoolConstant(false)
+Evaluated with empty environment: EqualsNull @ org-dartlang-testcase:///issue56803.dart:8:22 -> BoolConstant(true)
+Evaluated with empty environment: StaticGet @ org-dartlang-testcase:///issue56803.dart:8:16 -> NullConstant(null)
+Evaluated with empty environment: ConditionalExpression @ org-dartlang-testcase:///issue56803.dart:5:57 -> NullConstant(null)
+Evaluated with empty environment: FactoryConstructorInvocation @ org-dartlang-testcase:///issue56803.dart:5:28 -> BoolConstant(false)
+Extra constant evaluation: evaluated: 11, effectively constant: 5
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.strong.transformed.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.transformed.expect
new file mode 100644
index 00000000000..9b8c26f083d
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.strong.transformed.expect
@@ -0,0 +1,45 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type Ext(core::String _) implements core::String {
+  abstract extension-type-member representation-field get _() → core::String;
+  constructor _ = self::Ext|constructor#_;
+  constructor tearoff _ = self::Ext|constructor#_#_#tearOff;
+  constructor • = self::Ext|constructor#;
+  constructor tearoff • = self::Ext|constructor#_#new#tearOff;
+}
+static const field core::String? maybe = #C5;
+static extension-type-member method Ext|constructor#_(core::String _) → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = _;
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#_#tearOff(core::String _) → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#_(_);
+static extension-type-member method Ext|constructor#() → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = "OK";
+  assert(!(#C5 == null), "Must not be null");
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#new#tearOff() → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#();
+static method main() → void {
+  self::Ext /* erasure=core::String */ c = #C6;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = eval /* from org-dartlang-testcase:///issue56803.dart */ const self::Ext|constructor#()
+}
+
+Extra constant evaluation status:
+Evaluated with empty environment: Not @ org-dartlang-testcase:///issue56803.dart:8:22 -> BoolConstant(false)
+Evaluated with empty environment: EqualsNull @ org-dartlang-testcase:///issue56803.dart:8:22 -> BoolConstant(true)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803.dart:8:16 -> NullConstant(null)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803.dart:5:57 -> NullConstant(null)
+Extra constant evaluation: evaluated: 13, effectively constant: 4
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.textual_outline.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.textual_outline.expect
new file mode 100644
index 00000000000..c43f36cd3d0
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.textual_outline.expect
@@ -0,0 +1,9 @@
+const String? maybe = bool.fromEnvironment("not there") ? "string" : null;
+
+extension type const Ext._(String _) implements String {
+  const Ext()
+      : assert(maybe != null, "Must not be null"),
+        _ = "OK";
+}
+
+void main() {}
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.textual_outline_modelled.expect
new file mode 100644
index 00000000000..c43f36cd3d0
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.textual_outline_modelled.expect
@@ -0,0 +1,9 @@
+const String? maybe = bool.fromEnvironment("not there") ? "string" : null;
+
+extension type const Ext._(String _) implements String {
+  const Ext()
+      : assert(maybe != null, "Must not be null"),
+        _ = "OK";
+}
+
+void main() {}
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.weak.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.weak.expect
new file mode 100644
index 00000000000..b8303d9949b
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.weak.expect
@@ -0,0 +1,38 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type Ext(core::String _) implements core::String {
+  abstract extension-type-member representation-field get _() → core::String;
+  constructor _ = self::Ext|constructor#_;
+  constructor tearoff _ = self::Ext|constructor#_#_#tearOff;
+  constructor • = self::Ext|constructor#;
+  constructor tearoff • = self::Ext|constructor#_#new#tearOff;
+}
+static const field core::String? maybe = #C5;
+static extension-type-member method Ext|constructor#_(core::String _) → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = _;
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#_#tearOff(core::String _) → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#_(_);
+static extension-type-member method Ext|constructor#() → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = "OK";
+  assert(!(#C5 == null), "Must not be null");
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#new#tearOff() → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#();
+static method main() → void {
+  self::Ext /* erasure=core::String */ c = #C6;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = eval /* from org-dartlang-testcase:///issue56803.dart */ const self::Ext|constructor#()
+}
diff --git a/pkg/front_end/testcases/dart2js/issue56803.dart.weak.transformed.expect b/pkg/front_end/testcases/dart2js/issue56803.dart.weak.transformed.expect
new file mode 100644
index 00000000000..9b8c26f083d
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803.dart.weak.transformed.expect
@@ -0,0 +1,45 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+extension type Ext(core::String _) implements core::String {
+  abstract extension-type-member representation-field get _() → core::String;
+  constructor _ = self::Ext|constructor#_;
+  constructor tearoff _ = self::Ext|constructor#_#_#tearOff;
+  constructor • = self::Ext|constructor#;
+  constructor tearoff • = self::Ext|constructor#_#new#tearOff;
+}
+static const field core::String? maybe = #C5;
+static extension-type-member method Ext|constructor#_(core::String _) → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = _;
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#_#tearOff(core::String _) → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#_(_);
+static extension-type-member method Ext|constructor#() → self::Ext /* erasure=core::String */ {
+  lowered final self::Ext /* erasure=core::String */ #this = "OK";
+  assert(!(#C5 == null), "Must not be null");
+  return #this;
+}
+static extension-type-member method Ext|constructor#_#new#tearOff() → self::Ext /* erasure=core::String */
+  return self::Ext|constructor#();
+static method main() → void {
+  self::Ext /* erasure=core::String */ c = #C6;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = eval /* from org-dartlang-testcase:///issue56803.dart */ const self::Ext|constructor#()
+}
+
+Extra constant evaluation status:
+Evaluated with empty environment: Not @ org-dartlang-testcase:///issue56803.dart:8:22 -> BoolConstant(false)
+Evaluated with empty environment: EqualsNull @ org-dartlang-testcase:///issue56803.dart:8:22 -> BoolConstant(true)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803.dart:8:16 -> NullConstant(null)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803.dart:5:57 -> NullConstant(null)
+Extra constant evaluation: evaluated: 13, effectively constant: 4
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart b/pkg/front_end/testcases/dart2js/issue56803b.dart
new file mode 100644
index 00000000000..47d9c696dc7
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart
@@ -0,0 +1,18 @@
+// Copyright (c) 2024, 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.
+
+const String? maybe = bool.fromEnvironment("not there") ? "string" : null;
+
+class Ext {
+  final String _;
+
+  const Ext()
+      : assert(maybe != null, "Must not be null"),
+        _ = "OK";
+}
+
+void main() {
+  var c = const Ext();
+  print(c);
+}
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.expect
new file mode 100644
index 00000000000..24c148afaf3
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.expect
@@ -0,0 +1,36 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Ext extends core::Object /*hasConstConstructor*/  {
+  final field core::String _;
+  const constructor •() → self::Ext
+    : assert(!(#C5 == null), "Must not be null"), self::Ext::_ = "OK", super core::Object::•()
+    ;
+  static method _#new#tearOff() → self::Ext
+    return new self::Ext::•();
+}
+static const field core::String? maybe = #C5;
+static method main() → void {
+  self::Ext c = #C10;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803b.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = "OK"
+  #C7 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C5 == null
+  #C8 = eval /* from org-dartlang-testcase:///issue56803b.dart */!#C7
+  #C9 = "Must not be null"
+  #C10 = eval /* from org-dartlang-testcase:///issue56803b.dart */ self::Ext{_:#C6, assert(#C8, #C9)}
+}
+
+
+Constructor coverage from constants:
+org-dartlang-testcase:///issue56803b.dart:
+- Ext. (from org-dartlang-testcase:///issue56803b.dart:10:9)
+- Object. (from org-dartlang-sdk:///lib/core/object.dart)
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.modular.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.modular.expect
new file mode 100644
index 00000000000..24c148afaf3
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.modular.expect
@@ -0,0 +1,36 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Ext extends core::Object /*hasConstConstructor*/  {
+  final field core::String _;
+  const constructor •() → self::Ext
+    : assert(!(#C5 == null), "Must not be null"), self::Ext::_ = "OK", super core::Object::•()
+    ;
+  static method _#new#tearOff() → self::Ext
+    return new self::Ext::•();
+}
+static const field core::String? maybe = #C5;
+static method main() → void {
+  self::Ext c = #C10;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803b.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = "OK"
+  #C7 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C5 == null
+  #C8 = eval /* from org-dartlang-testcase:///issue56803b.dart */!#C7
+  #C9 = "Must not be null"
+  #C10 = eval /* from org-dartlang-testcase:///issue56803b.dart */ self::Ext{_:#C6, assert(#C8, #C9)}
+}
+
+
+Constructor coverage from constants:
+org-dartlang-testcase:///issue56803b.dart:
+- Ext. (from org-dartlang-testcase:///issue56803b.dart:10:9)
+- Object. (from org-dartlang-sdk:///lib/core/object.dart)
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.outline.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.outline.expect
new file mode 100644
index 00000000000..b7465b87a18
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.outline.expect
@@ -0,0 +1,24 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Ext extends core::Object /*hasConstConstructor*/  {
+  final field core::String _;
+  const constructor •() → self::Ext
+    : assert(!(self::maybe == null), "Must not be null"), self::Ext::_ = "OK", super core::Object::•()
+    ;
+  static method _#new#tearOff() → self::Ext
+    return new self::Ext::•();
+}
+static const field core::String? maybe = const core::bool::fromEnvironment("not there") ?{core::String?} "string" : null;
+static method main() → void
+  ;
+
+
+Extra constant evaluation status:
+Evaluated with empty environment: Not @ org-dartlang-testcase:///issue56803b.dart:11:22 -> BoolConstant(false)
+Evaluated with empty environment: EqualsNull @ org-dartlang-testcase:///issue56803b.dart:11:22 -> BoolConstant(true)
+Evaluated with empty environment: StaticGet @ org-dartlang-testcase:///issue56803b.dart:11:16 -> NullConstant(null)
+Evaluated with empty environment: ConditionalExpression @ org-dartlang-testcase:///issue56803b.dart:5:57 -> NullConstant(null)
+Evaluated with empty environment: FactoryConstructorInvocation @ org-dartlang-testcase:///issue56803b.dart:5:28 -> BoolConstant(false)
+Extra constant evaluation: evaluated: 6, effectively constant: 5
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.transformed.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.transformed.expect
new file mode 100644
index 00000000000..9210ccfbd89
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.strong.transformed.expect
@@ -0,0 +1,43 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Ext extends core::Object /*hasConstConstructor*/  {
+  final field core::String _;
+  const constructor •() → self::Ext
+    : assert(!(#C5 == null), "Must not be null"), self::Ext::_ = "OK", super core::Object::•()
+    ;
+  static method _#new#tearOff() → self::Ext
+    return new self::Ext::•();
+}
+static const field core::String? maybe = #C5;
+static method main() → void {
+  self::Ext c = #C10;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803b.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = "OK"
+  #C7 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C5 == null
+  #C8 = eval /* from org-dartlang-testcase:///issue56803b.dart */!#C7
+  #C9 = "Must not be null"
+  #C10 = eval /* from org-dartlang-testcase:///issue56803b.dart */ self::Ext{_:#C6, assert(#C8, #C9)}
+}
+
+Extra constant evaluation status:
+Evaluated with empty environment: Not @ org-dartlang-testcase:///issue56803b.dart:11:22 -> BoolConstant(false)
+Evaluated with empty environment: EqualsNull @ org-dartlang-testcase:///issue56803b.dart:11:22 -> BoolConstant(true)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803b.dart:11:16 -> NullConstant(null)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803b.dart:5:57 -> NullConstant(null)
+Extra constant evaluation: evaluated: 8, effectively constant: 4
+
+
+Constructor coverage from constants:
+org-dartlang-testcase:///issue56803b.dart:
+- Ext. (from org-dartlang-testcase:///issue56803b.dart:10:9)
+- Object. (from org-dartlang-sdk:///lib/core/object.dart)
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.textual_outline.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.textual_outline.expect
new file mode 100644
index 00000000000..e7ad3cce97d
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.textual_outline.expect
@@ -0,0 +1,10 @@
+const String? maybe = bool.fromEnvironment("not there") ? "string" : null;
+
+class Ext {
+  final String _;
+  const Ext()
+      : assert(maybe != null, "Must not be null"),
+        _ = "OK";
+}
+
+void main() {}
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.textual_outline_modelled.expect
new file mode 100644
index 00000000000..d0b3fb7134f
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.textual_outline_modelled.expect
@@ -0,0 +1,10 @@
+class Ext {
+  const Ext()
+      : assert(maybe != null, "Must not be null"),
+        _ = "OK";
+  final String _;
+}
+
+const String? maybe = bool.fromEnvironment("not there") ? "string" : null;
+
+void main() {}
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.weak.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.weak.expect
new file mode 100644
index 00000000000..24c148afaf3
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.weak.expect
@@ -0,0 +1,36 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Ext extends core::Object /*hasConstConstructor*/  {
+  final field core::String _;
+  const constructor •() → self::Ext
+    : assert(!(#C5 == null), "Must not be null"), self::Ext::_ = "OK", super core::Object::•()
+    ;
+  static method _#new#tearOff() → self::Ext
+    return new self::Ext::•();
+}
+static const field core::String? maybe = #C5;
+static method main() → void {
+  self::Ext c = #C10;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803b.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = "OK"
+  #C7 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C5 == null
+  #C8 = eval /* from org-dartlang-testcase:///issue56803b.dart */!#C7
+  #C9 = "Must not be null"
+  #C10 = eval /* from org-dartlang-testcase:///issue56803b.dart */ self::Ext{_:#C6, assert(#C8, #C9)}
+}
+
+
+Constructor coverage from constants:
+org-dartlang-testcase:///issue56803b.dart:
+- Ext. (from org-dartlang-testcase:///issue56803b.dart:10:9)
+- Object. (from org-dartlang-sdk:///lib/core/object.dart)
diff --git a/pkg/front_end/testcases/dart2js/issue56803b.dart.weak.transformed.expect b/pkg/front_end/testcases/dart2js/issue56803b.dart.weak.transformed.expect
new file mode 100644
index 00000000000..9210ccfbd89
--- /dev/null
+++ b/pkg/front_end/testcases/dart2js/issue56803b.dart.weak.transformed.expect
@@ -0,0 +1,43 @@
+library;
+import self as self;
+import "dart:core" as core;
+
+class Ext extends core::Object /*hasConstConstructor*/  {
+  final field core::String _;
+  const constructor •() → self::Ext
+    : assert(!(#C5 == null), "Must not be null"), self::Ext::_ = "OK", super core::Object::•()
+    ;
+  static method _#new#tearOff() → self::Ext
+    return new self::Ext::•();
+}
+static const field core::String? maybe = #C5;
+static method main() → void {
+  self::Ext c = #C10;
+  core::print(c);
+}
+
+constants  {
+  #C1 = "not there"
+  #C2 = eval /* from org-dartlang-testcase:///issue56803b.dart */ const core::bool::fromEnvironment(#C1)
+  #C3 = "string"
+  #C4 = null
+  #C5 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C2 ?{core::String?} #C3 : #C4
+  #C6 = "OK"
+  #C7 = eval /* from org-dartlang-testcase:///issue56803b.dart */ #C5 == null
+  #C8 = eval /* from org-dartlang-testcase:///issue56803b.dart */!#C7
+  #C9 = "Must not be null"
+  #C10 = eval /* from org-dartlang-testcase:///issue56803b.dart */ self::Ext{_:#C6, assert(#C8, #C9)}
+}
+
+Extra constant evaluation status:
+Evaluated with empty environment: Not @ org-dartlang-testcase:///issue56803b.dart:11:22 -> BoolConstant(false)
+Evaluated with empty environment: EqualsNull @ org-dartlang-testcase:///issue56803b.dart:11:22 -> BoolConstant(true)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803b.dart:11:16 -> NullConstant(null)
+Evaluated with empty environment: ConstantExpression @ org-dartlang-testcase:///issue56803b.dart:5:57 -> NullConstant(null)
+Extra constant evaluation: evaluated: 8, effectively constant: 4
+
+
+Constructor coverage from constants:
+org-dartlang-testcase:///issue56803b.dart:
+- Ext. (from org-dartlang-testcase:///issue56803b.dart:10:9)
+- Object. (from org-dartlang-sdk:///lib/core/object.dart)
diff --git a/tests/web/extension_type_assert_error_test.dart b/tests/web/extension_type_assert_error_test.dart
new file mode 100644
index 00000000000..34bd65071e3
--- /dev/null
+++ b/tests/web/extension_type_assert_error_test.dart
@@ -0,0 +1,21 @@
+// Copyright (c) 2024, 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.
+
+// dart2jsOptions=--enable-asserts
+
+import "package:expect/expect.dart";
+
+const String? maybe = bool.fromEnvironment("not there") ? "string" : null;
+
+extension type const Ext._(String _) implements String {
+  const Ext()
+      : assert(maybe != null, "Must not be null"),
+        _ = "OK";
+}
+
+void main() {
+  const Ext();
+  //    ^
+  // [cfe] Constant evaluation error:
+}