Add a scanner and parser for platform selectors.

lib/src/backend/platform_selector/token.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 unittest.backend.platform_selector.token;
+
+import 'package:source_span/source_span.dart';
+
+/// A token in a platform selector.
+class Token {
+  /// The type of the token.
+  final TokenType type;
+
+  /// The span indicating where this token came from.
+  ///
+  /// This is a [FileSpan] because the tokens are parsed from a single
+  /// continuous string, but the string itself isn't actually a file. It might

[Bob Nystrom, 2015/03/11 20:05:52]

This is confusing. Does that mean this should *not* be a FileSpan?

[nweiz, 2015/03/12 19:48:57]

No; "FileSpan" is somewhat of a misnomer. It just means a span that's parsed
from a single contiguous block of text.

+  /// come from a statically-parsed annotation or from a parameter.
+  final FileSpan span;
+
+  Token(this.type, this.span);
+}
+
+/// An token representing an identifier.

[Bob Nystrom, 2015/03/11 20:05:53]

"An" -> "A".

[nweiz, 2015/03/12 19:48:57]

Done.

+class IdentifierToken implements Token {
+  final type = TokenType.identifier;
+  final FileSpan span;
+
+  /// The name of the identifier.
+  final String name;
+
+  IdentifierToken(this.name, this.span);
+
+  String toString() => 'identifier "$name"';
+}
+
+/// An enumeration of types of tokens.
+class TokenType {

[Bob Nystrom, 2015/03/11 20:05:53]

Sigh, yet another case where enum should be used but can't because the feature
was too limited. :(

+  /// A `(` character.
+  static const leftParen = const TokenType._("left paren");
+
+  /// A `)` character.
+  static const rightParen = const TokenType._("right paren");
+
+  /// A `||` sequence.
+  static const or = const TokenType._("or");
+
+  /// A `&&` sequence.
+  static const and = const TokenType._("and");
+
+  /// A `!` character.
+  static const not = const TokenType._("not");
+
+  /// A `?` character.
+  static const questionMark = const TokenType._("question mark");
+
+  /// A `:` character.
+  static const colon = const TokenType._("colon");
+
+  /// A named identifier.
+  static const identifier = const TokenType._("identifier");
+
+  /// The end of the selector.
+  static const endOfFile = const TokenType._("end of file");
+
+  /// The name of the token type.
+  final String name;
+
+  const TokenType._(this.name);
+
+  String toString() => name;
+}