Tokenize one line at a time.

dart/site/try/line_numbers.css

Index: dart/site/try/line_numbers.css
diff --git a/dart/site/try/line_numbers.css b/dart/site/try/line_numbers.css
new file mode 100644
index 0000000000000000000000000000000000000000..7578c9bcab9c0e5f80b97ed0ede077f7e214ba81
--- /dev/null
+++ b/dart/site/try/line_numbers.css
@@ -0,0 +1,26 @@
+/* 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.
+ */
+
+.mainEditorPane {
+  padding: 0px;
+}
+
+div {
+  counter-reset: dart-line-number;
+}
+
+.lineNumber {
+  padding-left: 3.5em;
+}
+
+.lineNumber:before {
+  counter-increment: dart-line-number;
+  content: counter(dart-line-number) " ";
+  position: absolute;
+  left: 0px;
+  width: 3em;
+  text-align: right;
+  background-color: lightgoldenrodyellow;
+}