[fixed] rootDir in tsconfig

Summary: As recommended by TS team in microsoft/TypeScript#12698 (comment)

Reviewers: #material_motion, O2 Material Motion, featherless

Reviewed By: #material_motion, O2 Material Motion, featherless

Tags: #material_motion

Differential Revision: http://codereview.cc/D2297

Author: appsforartists

tsconfig.json

@@ -14,6 +14,7 @@
     "noUnusedLocals": true,
     "noUnusedParameters": true,
     "outDir": "dist",
+    "rootDir": "src",
     "sourceMap": true,
     "strictNullChecks": true,
     "target": "es6"