Avoid using RegExps in path to work around issue 19090.

pkg/path/lib/src/context.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.context;
 
 import 'internal_style.dart';
 import 'style.dart';
 import 'parsed_path.dart';
 import 'path_exception.dart';
@@ skipping 201 matching lines @@
     var buffer = new StringBuffer();
     var needsSeparator = false;
     var isAbsoluteAndNotRootRelative = false;
 
     for (var part in parts.where((part) => part != '')) {
       if (this.isRootRelative(part) && isAbsoluteAndNotRootRelative) {
         // If the new part is root-relative, it preserves the previous root but
         // replaces the path after it.
         var parsed = _parse(part);
         parsed.root = this.rootPrefix(buffer.toString());
-        if (parsed.root.contains(style.needsSeparatorPattern)) {
+        if (style.needsSeparator(parsed.root)) {
           parsed.separators[0] = style.separator;
         }
         buffer.clear();
         buffer.write(parsed.toString());
       } else if (this.isAbsolute(part)) {
         isAbsoluteAndNotRootRelative = !this.isRootRelative(part);
         // An absolute path discards everything before it.
         buffer.clear();
         buffer.write(part);
       } else {
-        if (part.length > 0 && part[0].contains(style.separatorPattern)) {
+        if (part.length > 0 && style.containsSeparator(part[0])) {
           // The part starts with a separator, so we don't need to add one.
         } else if (needsSeparator) {
           buffer.write(separator);
         }
 
         buffer.write(part);
       }
 
       // Unless this part ends with a separator, we'll need to add one before
       // the next part.
-      needsSeparator = part.contains(style.needsSeparatorPattern);
+      needsSeparator = style.needsSeparator(part);
     }
 
     return buffer.toString();
   }
 
   // TODO(nweiz): add a UNC example for Windows once issue 7323 is fixed.
   /// Splits [path] into its components using the current platform's
   /// [separator]. Example:
   ///
   ///     context.split('path/to/foo'); // -> ['path', 'to', 'foo']
@@ skipping 283 matching lines @@
     // Show the arguments.
     var message = new StringBuffer();
     message.write("$method(");
     message.write(args.take(numArgs)
         .map((arg) => arg == null ? "null" : '"$arg"')
         .join(", "));
     message.write("): part ${i - 1} was null, but part $i was not.");
     throw new ArgumentError(message.toString());
   }
 }