Rewrite the pkg/yaml parser.

pkg/string_scanner/test/string_scanner_test.dart

 // Copyright (c) 2014, 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 string_scanner.string_scanner_test;
 
 import 'package:string_scanner/string_scanner.dart';
 import 'package:unittest/unittest.dart';
 
 void main() {
@@ skipping 43 matching lines @@
       expect(scanner.lastMatch, isNull);
       expect(scanner.position, equals(0));
     });
 
     test("matches returns false and doesn't change the state", () {
       expect(scanner.matches(new RegExp('.')), isFalse);
       expect(scanner.lastMatch, isNull);
       expect(scanner.position, equals(0));
     });
 
+    test("substring returns the empty string", () {
+      expect(scanner.substring(0), isEmpty);
+    });
+
     test('setting position to 1 throws an ArgumentError', () {
       expect(() {
         scanner.position = 1;
       }, throwsArgumentError);
     });
 
     test('setting position to -1 throws an ArgumentError', () {
       expect(() {
         scanner.position = -1;
       }, throwsArgumentError);
@@ skipping 84 matching lines @@
     });
 
     test("a non-matching matches returns false and doesn't change the state",
         () {
       expect(scanner.matches(new RegExp('b(..)')), isFalse);
       expect(scanner.lastMatch, isNull);
       expect(scanner.position, equals(0));
       expect(scanner.rest, equals('foo bar'));
     });
 
+    test("substring from the beginning returns the empty string", () {
+      expect(scanner.substring(0), isEmpty);
+    });
+
+    test("substring with a custom end returns the substring", () {
+      expect(scanner.substring(0, 3), equals('foo'));
+    });
+
+    test("substring with the string length returns the whole string", () {
+      expect(scanner.substring(0, 7), equals('foo bar'));
+    });
+
     test('setting position to 1 moves the cursor forward', () {
       scanner.position = 1;
       expect(scanner.position, equals(1));
       expect(scanner.rest, equals('oo bar'));
 
       expect(scanner.scan(new RegExp('oo.')), isTrue);
       expect(scanner.lastMatch[0], equals('oo '));
       expect(scanner.position, equals(4));
       expect(scanner.rest, equals('bar'));
     });
@@ skipping 75 matching lines @@
       expect(scanner.lastMatch, isNull);
       expect(scanner.position, equals(7));
     });
 
     test("matches returns false sets lastMatch to null", () {
       expect(scanner.matches(new RegExp('.')), isFalse);
       expect(scanner.lastMatch, isNull);
       expect(scanner.position, equals(7));
     });
 
+    test("substring from the beginning returns the whole string", () {
+      expect(scanner.substring(0), equals('foo bar'));
+    });
+
+    test("substring with a custom start returns a substring from there", () {
+      expect(scanner.substring(4), equals('bar'));
+    });
+
+    test("substring with a custom start and end returns that substring", () {
+      expect(scanner.substring(3, 5), equals(' b'));
+    });
+
     test('setting position to 1 moves the cursor backward', () {
       scanner.position = 1;
       expect(scanner.position, equals(1));
       expect(scanner.rest, equals('oo bar'));
 
       expect(scanner.scan(new RegExp('oo.')), isTrue);
       expect(scanner.lastMatch[0], equals('oo '));
       expect(scanner.position, equals(4));
       expect(scanner.rest, equals('bar'));
     });
@@ skipping 27 matching lines @@
       expect(() => new StringScanner('foo bar', position: -1),
           throwsArgumentError);
     });
 
     test('throws an ArgumentError if the position is beyond the string', () {
       expect(() => new StringScanner('foo bar', position: 8),
           throwsArgumentError);
     });
   });
 }