Add a string_scanner package.

pkg/string_scanner/README.md

+This package exposes a `StringScanner` type that makes it easy to parse a string
+using a series of `Pattern`s. For example:
+
+```dart
+import 'dart:math';
+
+import 'package:string_scanner/string_scanner.dart';
+
+num parseNumber(String source) {
+  // Scan a number ("1", "1.5", "-3").
+  var scanner = new 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(new 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]);
+
+  if (scanner.scan(".")) {
+    scanner.expect(new 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;
+}
+```