Fix all strong mode warnings.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1843133004 .
diff --git a/.analysis_options b/.analysis_options
new file mode 100644
index 0000000..a10d4c5
--- /dev/null
+++ b/.analysis_options
@@ -0,0 +1,2 @@
+analyzer:
+  strong-mode: true
diff --git a/CHANGELOG.md b/CHANGELOG.md
index b505230..78c85e7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+# 1.2.4
+
+* Fix all remaining strong mode warnings.
+
 # 1.2.3
 
 * Addressed three strong mode warnings.
diff --git a/lib/src/version.dart b/lib/src/version.dart
index 528c3a2..9872289 100644
--- a/lib/src/version.dart
+++ b/lib/src/version.dart
@@ -4,7 +4,7 @@
 
 import 'dart:math' as math;
 
-import 'package:collection/equality.dart';
+import 'package:collection/collection.dart';
 
 import 'patterns.dart';
 import 'version_constraint.dart';
diff --git a/lib/src/version_union.dart b/lib/src/version_union.dart
index 2799341..5d52564 100644
--- a/lib/src/version_union.dart
+++ b/lib/src/version_union.dart
@@ -55,7 +55,7 @@
       throw new ArgumentError('Unknown VersionConstraint type $constraint.');
     }
 
-    (flattened as List).sort(compareMax);
+    flattened.sort(compareMax);
 
     var merged = <VersionRange>[];
     for (var constraint in flattened) {
diff --git a/pubspec.yaml b/pubspec.yaml
index 0ed3e3b..a026e25 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: pub_semver
-version: 1.2.3
+version: 1.2.4
 author: Dart Team <misc@dartlang.org>
 description: >
  Versions and version constraints implementing pub's versioning policy. This