Added library comment and warning message to web_sql.

sdk/lib/web_sql/dart2js/web_sql_dart2js.dart

Index: sdk/lib/web_sql/dart2js/web_sql_dart2js.dart
diff --git a/sdk/lib/web_sql/dart2js/web_sql_dart2js.dart b/sdk/lib/web_sql/dart2js/web_sql_dart2js.dart
index 572d96395b76229963d2fc6e20beb19d77611d0d..ba8d14e3321d213d6820be0f5ebad4b34cdf1d28 100644
--- a/sdk/lib/web_sql/dart2js/web_sql_dart2js.dart
+++ b/sdk/lib/web_sql/dart2js/web_sql_dart2js.dart
@@ -1,3 +1,14 @@
+/**
+ * An API for storing data in the browser that can be queried with SQL.
+ *
+ * **Caution:** this specification is no longer actively maintained by the Web
+ * Applications Working Group and may be removed at any time.
+ * See [the W3C Web SQL Database specification](http://www.w3.org/TR/webdatabase/)
+ * for more information.
+ *
+ * The [dart:html.Storage] or [dart:indexed_db] APIs are recommended
+ * alternatives.
+ */
 library dart.dom.web_sql;
 
 import 'dart:async';