Avoid using RegExps in path to work around issue 19090.

pkg/path/lib/src/style/posix.dart

Index: pkg/path/lib/src/style/posix.dart
diff --git a/pkg/path/lib/src/style/posix.dart b/pkg/path/lib/src/style/posix.dart
index e0b601743c743b829aa817fc5721e3128cadb9a9..b8b82b406d33f31a08d5c4a05c67b86dbdbc4b2f 100644
--- a/pkg/path/lib/src/style/posix.dart
+++ b/pkg/path/lib/src/style/posix.dart
@@ -4,6 +4,7 @@
 
 library path.style.posix;
 
+import '../characters.dart' as chars;
 import '../parsed_path.dart';
 import '../internal_style.dart';
 
@@ -13,9 +14,28 @@ class PosixStyle extends InternalStyle {
 
   final name = 'posix';
   final separator = '/';
+  final separators = const ['/'];
+
+  // Deprecated properties.
+
   final separatorPattern = new RegExp(r'/');
   final needsSeparatorPattern = new RegExp(r'[^/]$');
   final rootPattern = new RegExp(r'^/');
+  final relativeRootPattern = null;
+
+  bool containsSeparator(String path) => path.contains('/');
+
+  bool isSeparator(int codeUnit) => codeUnit == chars.SLASH;
+
+  bool needsSeparator(String path) =>
+      path.isNotEmpty && !isSeparator(path.codeUnitAt(path.length - 1));
+
+  String getRoot(String path) {
+    if (path.isNotEmpty && isSeparator(path.codeUnitAt(0))) return '/';
+    return null;
+  }
+
+  String getRelativeRoot(String path) => null;
 
   String pathFromUri(Uri uri) {
     if (uri.scheme == '' || uri.scheme == 'file') {