diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0949abb..3f7ea76 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -47,7 +47,7 @@ jobs:
       matrix:
         # Add macos-latest and/or windows-latest if relevant for this package.
         os: [ubuntu-latest]
-        sdk: [3.0, dev]
+        sdk: [3.4, dev]
     steps:
       - uses: actions/checkout@d632683dd7b4114ad314bca15554477dd762a938
       - uses: dart-lang/setup-dart@0a8a0fc875eb934c15d08629302413c671d3f672
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 30d9ef6..afe767d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 0.12.17-wip
+
+* Require Dart 3.4
+
 ## 0.12.16+1
 
 * Require Dart 3.0
diff --git a/lib/src/custom_matcher.dart b/lib/src/custom_matcher.dart
index 4fd53e0..b0f2d6b 100644
--- a/lib/src/custom_matcher.dart
+++ b/lib/src/custom_matcher.dart
@@ -8,7 +8,9 @@ import 'description.dart';
 import 'interfaces.dart';
 import 'util.dart';
 
-/// A useful utility class for implementing other matchers through inheritance.
+/// A base class for [Matcher] instances that match based on some feature of the
+/// value under test.
+///
 /// Derived classes should call the base constructor with a feature name and
 /// description, and an instance matcher, and should implement the
 /// [featureValueOf] abstract method.
diff --git a/lib/src/expect/expect_async.dart b/lib/src/expect/expect_async.dart
index 7571383..88cf6f2 100644
--- a/lib/src/expect/expect_async.dart
+++ b/lib/src/expect/expect_async.dart
@@ -203,7 +203,7 @@ class _ExpectedFunction<T> {
   void _afterRun() {
     if (_complete) return;
     if (_minExpectedCalls > 0 && _actualCalls < _minExpectedCalls) return;
-    if (_isDone != null && !_isDone!()) return;
+    if (_isDone != null && !_isDone()) return;
 
     // Mark this callback as complete and remove it from the test case's
     // outstanding callback count; if that hits zero the test is done.
diff --git a/lib/src/expect/future_matchers.dart b/lib/src/expect/future_matchers.dart
index e1328b6..407b9b8 100644
--- a/lib/src/expect/future_matchers.dart
+++ b/lib/src/expect/future_matchers.dart
@@ -55,12 +55,12 @@ class _Completes extends AsyncMatcher {
 
       String? result;
       if (_matcher is AsyncMatcher) {
-        result = await (_matcher as AsyncMatcher).matchAsync(value) as String?;
+        result = await _matcher.matchAsync(value) as String?;
         if (result == null) return null;
       } else {
         var matchState = {};
-        if (_matcher!.matches(value, matchState)) return null;
-        result = _matcher!
+        if (_matcher.matches(value, matchState)) return null;
+        result = _matcher
             .describeMismatch(value, StringDescription(), matchState, false)
             .toString();
       }
diff --git a/lib/src/expect/throws_matcher.dart b/lib/src/expect/throws_matcher.dart
index 0ee3144..3a81821 100644
--- a/lib/src/expect/throws_matcher.dart
+++ b/lib/src/expect/throws_matcher.dart
@@ -121,9 +121,9 @@ class Throws extends AsyncMatcher {
     if (_matcher == null) return null;
 
     var matchState = {};
-    if (_matcher!.matches(error, matchState)) return null;
+    if (_matcher.matches(error, matchState)) return null;
 
-    var result = _matcher!
+    var result = _matcher
         .describeMismatch(error, StringDescription(), matchState, false)
         .toString();
 
diff --git a/pubspec.yaml b/pubspec.yaml
index 8b6df88..938a18a 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,12 +1,12 @@
 name: matcher
-version: 0.12.16+1
+version: 0.12.17-wip
 description: >-
   Support for specifying test expectations via an extensible Matcher class.
   Also includes a number of built-in Matcher implementations for common cases.
 repository: https://github.com/dart-lang/matcher
 
 environment:
-  sdk: ^3.0.0
+  sdk: ^3.4.0
 
 dependencies:
   async: ^2.10.0
@@ -19,3 +19,7 @@ dev_dependencies:
   fake_async: ^1.3.0
   lints: ^3.0.0
   test: ^1.23.0
+
+dependency_overrides:
+  test: 1.25.0
+  test_api: 0.7.3