Add a scanner and parser for platform selectors.

lib/src/backend/platform_selector/parser.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.parser;
+
+import 'package:source_span/source_span.dart';
+
+import 'ast.dart';
+import 'scanner.dart';
+import 'token.dart';
+
+/// A class for parsing a platform selector.
+///
+/// Platform selectors use a stripped-down version of the Dart expression
+/// syntax that only contains variables, parentheses, and boolean operators.
+/// Variables may also contain dashes, contrary to Dart's syntax; this allows
+/// consistency with command-line arguments.

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

Does this add enough value to be worth the possible confusion?

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

I think so. A lot of the identifiers that people will be using here will match
the identifiers passed on the command line, and having to remember to switch
from hyphens on the command line to underscores here (or, if you split it
differently, switch from hyphens for command-line flags to underscores for their
values) will be really painful. Dart identifiers, on the other hand, aren't
continuous with this syntax; there aren't Dart values that need to be translated
between the two, so there's less friction.

+class Parser {
+  /// The scanner that tokenizes the selector.
+  final Scanner _scanner;
+
+  Parser(String selector)
+      : _scanner = new Scanner(selector);
+
+  /// Parses the selector.
+  ///
+  /// This must only be called once per parser.
+  Node parse() {
+    var selector = _conditional();
+
+    if (_scanner.peek().type != TokenType.endOfFile) {
+      throw new SourceSpanFormatException(
+          "Expected end of input.", _scanner.peek().span);
+    }
+
+    return selector;
+  }
+
+  /// Parses a conditional:
+  ///
+  ///     conditionalExpression:
+  ///       logicalOrExpression ("?" conditionalExpression ":"
+  ///           conditionalExpression)?
+  Node _conditional() {
+    var condition = _or();
+    if (_scanner.peek().type != TokenType.questionMark) return condition;
+
+    _scanner.next();

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

You could simplify this (and a bunch below) by adding a match() method to
Scanner that combines the peek and next:

if (!_scanner.match(TokenType.questionMark) return condition;

...

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

Done.

+    var branch1 = _conditional();
+    var token = _scanner.next();
+    if (token.type != TokenType.colon) {
+      throw new SourceSpanFormatException('Expected ":".', token.span);
+    }
+
+    var branch2 = _conditional();
+    return new ConditionalNode(condition, branch1, branch2);
+  }
+
+  /// Parses a logical or:
+  ///
+  ///     logicalOrExpression:
+  ///       logicalAndExpression ("||" logicalOrExpression)?
+  Node _or() {
+    var branch1 = _and();
+    if (_scanner.peek().type != TokenType.or) return branch1;
+    _scanner.next();
+    return new OrNode(branch1, _or());
+  }
+
+  /// Parses a logical and:
+  ///
+  ///     logicalAndExpression:
+  ///       simpleExpression ("&&" logicalAndExpression)?
+  Node _and() {
+    var branch1 = _simpleExpression();
+    if (_scanner.peek().type != TokenType.and) return branch1;
+    _scanner.next();
+    return new AndNode(branch1, _and());
+  }
+
+  /// Parses a simple expression:
+  ///
+  ///     simpleExpression:
+  ///       "!" simpleExpression |
+  ///           "(" conditionalExpression ")" |
+  ///           IDENTIFIER
+  Node _simpleExpression() {
+    var token = _scanner.next();
+    switch (token.type) {
+      case TokenType.not:
+        var child = _simpleExpression();
+        return new NotNode(child, token.span.expand(child.span));
+
+      case TokenType.leftParen:
+        var child = _conditional();
+        var next = _scanner.next();
+        if (next.type != TokenType.rightParen) {
+          throw new SourceSpanFormatException('Expected ")".', next.span);
+        }
+        return child;
+
+      case TokenType.identifier:
+        return new VariableNode(token.name, token.span);
+
+      default:
+        throw new SourceSpanFormatException("Expected expression.", token.span);
+    }
+  }
+}