Port dart:io Path tests to package:path.

pkg/path/lib/path.dart

Index: pkg/path/lib/path.dart
diff --git a/pkg/path/lib/path.dart b/pkg/path/lib/path.dart
index 25ce0e6fb850199240b4d00d90f72bd92a12449c..e56eb596b03e01c697ce140fa3cc2f2b6f5943f2 100644
--- a/pkg/path/lib/path.dart
+++ b/pkg/path/lib/path.dart
@@ -114,6 +114,17 @@ String basenameWithoutExtension(String path) =>
 /// Trailing separators are ignored.
 ///
 ///     builder.dirname('path/to/'); // -> 'path'
+///
+/// If an absolute path contains no directories, only a root, then the root
+/// is returned.
+///
+///     path.dirname('/');  // -> '/' (posix)
+///     path.dirname('c:\');  // -> 'c:\' (windows)
+///
+/// If a relative path has no directories, then '.' is returned.
+///
+///     path.dirname('foo');  // -> '.'
+///     path.dirname('');  // -> '.'
 String dirname(String path) => _builder.dirname(path);
 
 /// Gets the file extension of [path]: the portion of [basename] from the last
@@ -588,8 +599,6 @@ class Builder {
   ///
   ///     builder.normalize('path/./to/..//file.text'); // -> 'path/file.txt'
   String normalize(String path) {
-    if (path == '') return path;
-
     var parsed = _parse(path);
     parsed.normalize();
     return parsed.toString();
@@ -630,8 +639,6 @@ class Builder {
   ///     var builder = new Builder(r'some/relative/path');
   ///     builder.relative(r'/absolute/path'); // -> '/absolute/path'
   String relative(String path, {String from}) {
-    if (path == '') return '.';
-
     from = from == null ? root : this.join(root, from);
 
     // We can't determine the path from a relative path to an absolute path.
@@ -678,8 +685,12 @@ class Builder {
       pathParsed.separators.removeAt(1);
     }
 
-    // If there are any directories left in the root path, we need to walk up
-    // out of them.
+    // If there are any directories left in the from path, we need to walk up
+    // out of them.  If a directory left in the from path is '..', it cannot
+    // be cancelled by adding a '..'.
+    if (fromParsed.parts.length > 0 && fromParsed.parts[0] == '..') {
+      throw new ArgumentError('Unable to find a path to "$path" from "$from".');
+    }
     _growListFront(pathParsed.parts, fromParsed.parts.length, '..');
     pathParsed.separators[0] = '';
     pathParsed.separators.insertAll(1,
@@ -688,6 +699,13 @@ class Builder {
     // Corner case: the paths completely collapsed.
     if (pathParsed.parts.length == 0) return '.';
 
+    // Corner case: path was '.' and some '..' directories were added in front.
+    // Don't add a final '/.' in that case.
+    if (pathParsed.parts.length > 1 && pathParsed.parts.last == '.') {
+      pathParsed.parts.removeLast();
+      pathParsed.separators..removeLast()..removeLast()..add('');
+    }
+
     // Make it relative.
     pathParsed.root = '';
     pathParsed.removeTrailingSeparators();
@@ -1046,7 +1064,8 @@ class _ParsedPath {
     return copy._splitExtension()[0];
   }
 
-  bool get hasTrailingSeparator => !parts.isEmpty && (parts.last == '' || separators.last != '');
+  bool get hasTrailingSeparator =>
+      !parts.isEmpty && (parts.last == '' || separators.last != '');
 
   void removeTrailingSeparators() {
     while (!parts.isEmpty && parts.last == '') {