Avoid using RegExps in path to work around issue 19090.

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

 // Copyright (c) 2013, 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.
 
 library path.style.windows;
 
+import '../characters.dart' as chars;
+import '../internal_style.dart';
 import '../parsed_path.dart';
-import '../internal_style.dart';
+import '../utils.dart';
 
 /// The style for Windows paths.
 class WindowsStyle extends InternalStyle {
   WindowsStyle();
 
   final name = 'windows';
   final separator = '\\';
+  final separators = const ['/', '\\'];
+
+  // Deprecated properties.
+
   final separatorPattern = new RegExp(r'[/\\]');
   final needsSeparatorPattern = new RegExp(r'[^/\\]$');
   final rootPattern = new RegExp(r'^(\\\\[^\\]+\\[^\\/]+|[a-zA-Z]:[/\\])');
   final relativeRootPattern = new RegExp(r"^[/\\](?![/\\])");
 
+  bool containsSeparator(String path) => path.contains('/');
+
+  bool isSeparator(int codeUnit) =>
+      codeUnit == chars.SLASH || codeUnit == chars.BACKSLASH;
+
+  bool needsSeparator(String path) {
+    if (path.isEmpty) return false;
+    return !isSeparator(path.codeUnitAt(path.length - 1));
+  }
+
+  String getRoot(String path) {
+    var root = _getRoot(path);
+    return root == null ? getRelativeRoot(path) : root;
+  }
+
+  String getRelativeRoot(String path) {
+    if (path.isEmpty) return null;
+    if (!isSeparator(path.codeUnitAt(0))) return null;
+    if (path.length > 1 && isSeparator(path.codeUnitAt(1))) return null;
+    return path[0];
+  }
+
   String pathFromUri(Uri uri) {
     if (uri.scheme != '' && uri.scheme != 'file') {
       throw new ArgumentError("Uri $uri must have scheme 'file:'.");
     }
 
     var path = uri.path;
     if (uri.host == '') {
       // Drive-letter paths look like "file:///C:/path/to/file". The
       // replaceFirst removes the extra initial slash.
       if (path.startsWith('/')) path = path.replaceFirst("/", "");
@@ skipping 28 matching lines @@
       // If the path is a bare root (e.g. "C:\"), [parsed.parts] will currently
       // be empty. We add an empty component so the URL constructor produces
       // "file:///C:/", with a trailing slash. We also add an empty component if
       // the URL otherwise has a trailing slash.
       if (parsed.parts.length == 0 || parsed.hasTrailingSeparator) {
         parsed.parts.add("");
       }
 
       // Get rid of the trailing "\" in "C:\" because the URI constructor will
       // add a separator on its own.
-      parsed.parts.insert(0, parsed.root.replaceAll(separatorPattern, ""));
+      parsed.parts.insert(0,
+          parsed.root.replaceAll("/", "").replaceAll("\\", ""));
 
       return new Uri(scheme: 'file', pathSegments: parsed.parts);
     }
   }
+
+  // A helper method for [getRoot] that doesn't handle relative roots.
+  String _getRoot(String path) {
+    if (path.length < 3) return null;
+
+    // We aren't using a RegExp for this because they're slow (issue 19090). If
+    // we could, we'd match against r'^(\\\\[^\\]+\\[^\\/]+|[a-zA-Z]:[/\\])'.
+
+    // Try roots like "C:\".
+    if (isAlphabetic(path.codeUnitAt(0))) {
+      if (path.codeUnitAt(1) != chars.COLON) return null;
+      if (!isSeparator(path.codeUnitAt(2))) return null;
+      return path.substring(0, 3);
+    }
+
+    // Try roots like "\\server\share".
+    if (!path.startsWith('\\\\')) return null;
+
+    var start = 2;
+    // The server is one or more non-"\" characters.
+    while (start < path.length && path.codeUnitAt(start) != chars.BACKSLASH) {
+      start++;
+    }
+    if (start == 2 || start == path.length) return null;
+
+    // The share is one or more non-"\" characters.
+    start += 1;
+    if (path.codeUnitAt(start) == chars.BACKSLASH) return null;
+    start += 1;
+    while (start < path.length && path.codeUnitAt(start) != chars.BACKSLASH) {
+      start++;
+    }
+
+    return path.substring(0, start);
+  }
 }