Add LineScanner and SpanScanner classes to string_scanner.

pkg/string_scanner/CHANGELOG.md

Index: pkg/string_scanner/CHANGELOG.md
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+## 0.0.2
+
+* `new StringScanner` now takes an optional `sourceUrl` argument that provides

[Bob Nystrom, 2014/05/28 21:28:57]

Nit, but a ctor call without () looks weird to me.

[nweiz, 2014/05/28 23:56:34]

Done.

+  the URL of the source file. This is used for error reporting.
+
+* Add `StringScanner.readChar` and `StringScanner.peekChar` methods for doing

[Bob Nystrom, 2014/05/28 21:28:57]

And here too, since otherwise a reader might think these are getters.

[nweiz, 2014/05/28 23:56:34]

Done.

+  character-by-character scanning.
+
+* Scanners now throw `StringScannerException`s which provide more detailed
+  access to information about the errors that were thrown and can provide
+  terminal-colored messages.
+
+* Add a `LineScanner` subclass of `StringScanner` that automatically tracks line
+  and column information of the text being scanned.
+
+* Add a `SpanScanner` subclass of `LineScanner` that exposes matched ranges as
+  [source map][] `Span` objects.
+
+[source_map]: http://pub.dartlang.org/packages/source_maps