Optimize work with absolute paths.

pkg/analyzer/lib/src/util/absolute_path.dart

+// Copyright (c) 2015, 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 analyzer.src.util.absolute_path;
+
+/// The class for manipulating absolute paths.
+class AbsolutePathContext {
+  final String separator;
+
+  AbsolutePathContext(this.separator);
+
+  /// Append the given relative [suffix] to the given absolute [parent].
+  ///
+  ///     context.append('/path/to', 'foo'); // -> '/path/to/foo'
+  ///
+  /// The given [suffix] cannot be an absolute path or use `..`.
+  String append(String parent, String suffix) {
+    return '$parent$separator$suffix';
+  }
+
+  /// Return the part of the absolute [path] after the last separator on the
+  /// context's platform.
+  ///
+  ///     context.basename('/path/to/foo.dart'); // -> 'foo.dart'
+  ///     context.basename('/path/to');          // -> 'to'
+  ///     context.basename('/path');             // -> 'path'
+  ///     context.basename('/');                 // -> ''
+  String basename(String path) {
+    int index = path.lastIndexOf(separator);
+    return path.substring(index + 1);
+  }
+
+  /// Return the part of the absolute [path] before the last separator.
+  ///
+  ///     context.dirname('/path/to/foo.dart'); // -> '/path/to'
+  ///     context.dirname('/path/to');          // -> '/path'
+  ///     context.dirname(r'/path');            // -> '/'
+  ///     context.dirname(r'/');                // -> '/'
+  ///     context.dirname(r'C:\path');          // -> 'C:\'
+  ///     context.dirname(r'C:\');              // -> 'C:\'
+  String dirname(String path) {
+    int firstIndex = path.indexOf(separator);
+    int lastIndex = path.lastIndexOf(separator);
+    return lastIndex == firstIndex
+        ? path.substring(0, firstIndex + 1)
+        : path.substring(0, lastIndex);
+  }
+
+  /// Return `true` if [child] is a path beneath [parent], and `false`
+  /// otherwise. Both the [child] and [parent] paths must be absolute paths.
+  ///
+  ///     context.isWithin('/root/path', '/root/path/a'); // -> true
+  ///     context.isWithin('/root/path', '/root/other');  // -> false
+  ///     context.isWithin('/root/path', '/root/path');   // -> false
+  bool isWithin(String parent, String child) {
+    return child.startsWith(parent + separator);
+  }
+
+  /// Split [path] into its components using [separator].
+  ///
+  ///     context.split('/path/to/foo'); // -> ['', 'path', 'to', 'foo']
+  List<String> split(String path) {
+    return path.split(separator);
+  }
+
+  /// If the given [child] is within the given [parent], then return the
+  /// relative path from [parent] to [child]. Otherwise return `null`. Both
+  /// the [child] and [parent] paths must be absolute paths.
+  ///
+  ///     context.relative('/root/path/a/b.dart', '/root/path'); // -> 'a/b.dart'
+  ///     context.relative('/root/other.dart', '/root/path');    // -> null
+  String suffix(String child, String parent) {
+    String parentPrefix = parent + separator;
+    if (child.startsWith(parentPrefix)) {
+      return child.substring(parentPrefix.length);
+    }
+    return null;
+  }
+}