Add a scanner and parser for platform selectors.

test/backend/platform_selector/ast_test.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.
+
+import 'package:unittest/unittest.dart';
+import 'package:unittest/src/backend/platform_selector/ast.dart';
+import 'package:unittest/src/backend/platform_selector/parser.dart';
+
+void main() {
+  group("toString() for", () {
+    test("a variable is its name", () {
+      _expectToString("foo");
+      _expectToString("a-b");
+    });
+
+    group("not", () {
+      test("doesn't parenthesize a variable", () => _expectToString("!a"));
+      test("doesn't parenthesize a nested not", () => _expectToString("!!a"));
+      test("parenthesizes an or", () => _expectToString("!(a || b)"));
+      test("parenthesizes an and", () => _expectToString("!(a && b)"));
+      test("parenthesizes a condition", () => _expectToString("!(a ? b : c)"));
+    });
+
+    group("or", () {
+      test("doesn't parenthesize variables", () => _expectToString("a || b"));
+      test("doesn't parenthesize nots", () => _expectToString("!a || !b"));
+
+      test("doesn't parenthesize ors", () {
+        _expectToString("a || b || c || d");
+        _expectToString("((a || b) || c) || d", "a || b || c || d");
+      });
+
+      test("parenthesizes ands", () =>
+          _expectToString("a && b || c && d", "(a && b) || (c && d)"));
+
+      test("parenthesizes conditions", () =>
+          _expectToString("(a ? b : c) || (e ? f : g)"));
+    });
+
+    group("and", () {
+      test("doesn't parenthesize variables", () => _expectToString("a && b"));
+      test("doesn't parenthesize nots", () => _expectToString("!a && !b"));
+
+      test("parenthesizes ors", () =>
+          _expectToString("(a || b) && (c || d)", "(a || b) && (c || d)"));
+
+      test("doesn't parenthesize ands", () {
+        _expectToString("a && b && c && d");
+        _expectToString("((a && b) && c) && d", "a && b && c && d");
+      });
+
+      test("parenthesizes conditions", () =>
+          _expectToString("(a ? b : c) && (e ? f : g)"));
+    });
+
+    group("conditional", () {
+      test("doesn't parenthesize variables", () =>
+          _expectToString("a ? b : c"));
+
+      test("doesn't parenthesize nots", () => _expectToString("!a ? !b : !c"));
+
+      test("doesn't parenthesize ors", () =>
+          _expectToString("a || b ? c || d : e || f"));
+
+      test("doesn't parenthesize ands", () =>
+          _expectToString("a && b ? c && d : e && f"));
+
+      test("parenthesizes non-trailing conditions", () {
+        _expectToString("(a ? b : c) ? (e ? f : g) : h ? i : j");
+        _expectToString("(a ? b : c) ? (e ? f : g) : (h ? i : j)",
+            "(a ? b : c) ? (e ? f : g) : h ? i : j");
+      });
+    });
+  });
+}
+
+void _expectToString(String selector, [String result]) {
+  if (result == null) result = selector;
+  expect(_toString(selector), equals(result),
+      reason: 'Expected toString of "$selector" to be "$result".');
+}
+
+String _toString(String selector) => new Parser(selector).parse().toString();