Added example/example.dart, update README to working, valid code

Fixes https://github.com/dart-lang/string_scanner/issues/9

pkgs/string_scanner/README.md

diff --git a/pkgs/string_scanner/README.md b/pkgs/string_scanner/README.md
index 90660fc..15aa242 100644
--- a/pkgs/string_scanner/README.md
+++ b/pkgs/string_scanner/README.md
@@ -8,7 +8,7 @@
 
 num parseNumber(String source) {
   // Scan a number ("1", "1.5", "-3").
-  var scanner = new StringScanner(source);
+  var scanner = StringScanner(source);
 
   // [Scanner.scan] tries to consume a [Pattern] and returns whether or not it
   // succeeded. It will move the scan pointer past the end of the pattern.
@@ -16,14 +16,14 @@
 
   // [Scanner.expect] consumes a [Pattern] and throws a [FormatError] if it
   // fails. Like [Scanner.scan], it will move the scan pointer forward.
-  scanner.expect(new RegExp(r"\d+"));
+  scanner.expect(RegExp(r"\d+"));
 
   // [Scanner.lastMatch] holds the [MatchData] for the most recent call to
   // [Scanner.scan], [Scanner.expect], or [Scanner.matches].
-  var number = int.parse(scanner.lastMatch[0]);
+  var number = num.parse(scanner.lastMatch[0]);
 
   if (scanner.scan(".")) {
-    scanner.expect(new RegExp(r"\d+"));
+    scanner.expect(RegExp(r"\d+"));
     var decimal = scanner.lastMatch[0];
     number += int.parse(decimal) / math.pow(10, decimal.length);
   }

pkgs/string_scanner/example/example.dart

diff --git a/pkgs/string_scanner/example/example.dart b/pkgs/string_scanner/example/example.dart
new file mode 100644
index 0000000..ddd9d3e
--- /dev/null
+++ b/pkgs/string_scanner/example/example.dart
@@ -0,0 +1,40 @@
+// Copyright (c) 2019, 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 'dart:math' as math;
+
+import 'package:string_scanner/string_scanner.dart';
+
+void main(List<String> args) {
+  print(parseNumber(args.single));
+}
+
+num parseNumber(String source) {
+  // Scan a number ("1", "1.5", "-3").
+  var scanner = StringScanner(source);
+
+  // [Scanner.scan] tries to consume a [Pattern] and returns whether or not it
+  // succeeded. It will move the scan pointer past the end of the pattern.
+  var negative = scanner.scan("-");
+
+  // [Scanner.expect] consumes a [Pattern] and throws a [FormatError] if it
+  // fails. Like [Scanner.scan], it will move the scan pointer forward.
+  scanner.expect(RegExp(r"\d+"));
+
+  // [Scanner.lastMatch] holds the [MatchData] for the most recent call to
+  // [Scanner.scan], [Scanner.expect], or [Scanner.matches].
+  var number = num.parse(scanner.lastMatch[0]);
+
+  if (scanner.scan(".")) {
+    scanner.expect(RegExp(r"\d+"));
+    var decimal = scanner.lastMatch[0];
+    number += int.parse(decimal) / math.pow(10, decimal.length);
+  }
+
+  // [Scanner.expectDone] will throw a [FormatError] if there's any input that
+  // hasn't yet been consumed.
+  scanner.expectDone();
+
+  return (negative ? -1 : 1) * number;
+}