Delete a bunch of packages that are now on GitHub.

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;
-}
-```