Add LineScanner and SpanScanner classes to string_scanner.

pkg/string_scanner/lib/src/utils.dart

Index: pkg/string_scanner/lib/src/utils.dart
diff --git a/pkg/string_scanner/lib/src/utils.dart b/pkg/string_scanner/lib/src/utils.dart
new file mode 100644
index 0000000000000000000000000000000000000000..f01332eeeeaf1fcacec68361b2c016c4051c36e0
--- /dev/null
+++ b/pkg/string_scanner/lib/src/utils.dart
@@ -0,0 +1,25 @@
+// 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.utils;
+
+/// Validates the arguments passed to [StringScanner.error].
+void validateErrorArgs(String string, Match match, int position, int length) {
+  if (match != null && (position != null || length != null)) {
+    throw new ArgumentError("Can't pass both match and position/length.");
+  }
+
+  if (position != null) {
+    if (position < 0) {
+      throw new RangeError("position must be greater than or equal to 0.");
+    } else if (position > string.length) {
+      throw new RangeError("position must be less than or equal to the "
+          "string length.");
+    }
+  }
+
+  if (length != null && length < 0) {
+    throw new RangeError("length must be greater than or equal to 0.");
+  }

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

Check for position + length going past the end?

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

Done.

+}