Drop unnecessary constructors (#105)

When a class has no other constructors it is unnecessary and
non-idiomatic to define the default empty constructor.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7c15e3e..264eb6a 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,5 @@
+## 1.8.1-dev
+
 ## 1.8.0
 
 * Stable release for null safety.
diff --git a/lib/src/style/posix.dart b/lib/src/style/posix.dart
index 59c8d32..7091865 100644
--- a/lib/src/style/posix.dart
+++ b/lib/src/style/posix.dart
@@ -8,8 +8,6 @@
 
 /// The style for POSIX paths.
 class PosixStyle extends InternalStyle {
-  PosixStyle();
-
   @override
   final name = 'posix';
   @override
diff --git a/lib/src/style/url.dart b/lib/src/style/url.dart
index 9daccd7..3ff87ce 100644
--- a/lib/src/style/url.dart
+++ b/lib/src/style/url.dart
@@ -8,8 +8,6 @@
 
 /// The style for URL paths.
 class UrlStyle extends InternalStyle {
-  UrlStyle();
-
   @override
   final name = 'url';
   @override
diff --git a/lib/src/style/windows.dart b/lib/src/style/windows.dart
index 3b7d98c..b7542c6 100644
--- a/lib/src/style/windows.dart
+++ b/lib/src/style/windows.dart
@@ -13,8 +13,6 @@
 
 /// The style for Windows paths.
 class WindowsStyle extends InternalStyle {
-  WindowsStyle();
-
   @override
   final name = 'windows';
   @override
diff --git a/pubspec.yaml b/pubspec.yaml
index 67a0d51..4fe70bc 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: path
-version: 1.8.0
+version: 1.8.1-dev
 
 description: >-
   A string-based path manipulation library. All of the path operations you know