Bump to linter 0.1.50


* migration of rules to use analyzer package `NodeLintRule` and `UnitLintRule` yielding significant performance gains all around
* specific performance improvements for `prefer_final_fields` (~6x)
* addressed no such method calls in `void_checks`
* improved lint reporting for various lints

Change-Id: I63896a5367e27241ef596408aea44830c8067c75
Reviewed-on: https://dart-review.googlesource.com/53861
Reviewed-by: Devon Carew <devoncarew@google.com>
diff --git a/DEPS b/DEPS
index 6564eee..b3f4985 100644
--- a/DEPS
+++ b/DEPS
@@ -98,7 +98,7 @@
   "intl_tag": "0.15.2",
   "jinja2_rev": "2222b31554f03e62600cd7e383376a7c187967a1",
   "json_rpc_2_tag": "2.0.6",
-  "linter_tag": "0.1.49",
+  "linter_tag": "0.1.50",
   "logging_tag": "0.11.3+1",
   "markdown_tag": "1.1.1",
   "matcher_tag": "0.12.1+4",