Tokenize one line at a time.

dart/site/try/index.html

Index: dart/site/try/index.html
diff --git a/dart/site/try/index.html b/dart/site/try/index.html
index 63cdacd032f6de54e63979927a2ff61fbab478a0..0d43889d64f493f9a6d4db8c1736bc8bc4b699fb 100644
--- a/dart/site/try/index.html
+++ b/dart/site/try/index.html
@@ -148,6 +148,32 @@ a:hover.diagnostic span {
   height: auto;
 }
 
+.mainEditorPane {
+  overflow-y: scroll;
+  overflow-x: hidden;
+  white-space: pre;
+  padding: 0px;
+  max-height: 80vh;
+}
+
+div {
+  counter-reset: dart-line-number;
+}
+
+.lineNumber {
+  position: relative;
+  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;
+}
 
 </style>