Convert files to typescript

The change converts the following files to typescript:

* elements/diff/gr-diff-mode-selector/gr-diff-mode-selector.ts

Change-Id: I3d677069eb4e30a49536d21ea8e702a2f2cbbc7a
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts
index 40f6a32..73f7426 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.ts
@@ -34,7 +34,7 @@
     id="sideBySideBtn"
     link=""
     has-tooltip=""
-    class$="[[_computeSelectedClass(mode, _VIEW_MODES.SIDE_BY_SIDE)]]"
+    class$="[[_computeSideBySideSelected(mode)]]"
     title="Side-by-side diff"
     on-click="_handleSideBySideTap"
   >
@@ -45,7 +45,7 @@
     link=""
     has-tooltip=""
     title="Unified diff"
-    class$="[[_computeSelectedClass(mode, _VIEW_MODES.UNIFIED)]]"
+    class$="[[_computeUnifiedSelected(mode)]]"
     on-click="_handleUnifiedTap"
   >
     <iron-icon icon="gr-icons:unified"></iron-icon>