Add immutable.js.

package.json

@@ -4,6 +4,7 @@
   "private": true,
   "dependencies": {
     "ical.js": "^1.2.2",
+    "immutable": "^4.0.0-rc.9",
     "isomorphic-fetch": "^2.1.1",
     "localforage": "^1.5.5",
     "material-ui": "^0.20.0",

tsconfig.json

@@ -2,7 +2,7 @@
   "compilerOptions": {
     "outDir": "build/dist",
     "module": "esnext",
-    "target": "es5",
+    "target": "es2015",
     "lib": ["es6", "dom"],
     "sourceMap": true,
     "allowJs": true,

yarn.lock

@@ -2529,6 +2529,10 @@ immediate@~3.0.5:
   version "3.0.6"
   resolved "https://registry.yarnpkg.com/immediate/-/immediate-3.0.6.tgz#9db1dbd0faf8de6fbe0f5dd5e56bb606280de69b"
 
+immutable@^4.0.0-rc.9:
+  version "4.0.0-rc.9"
+  resolved "https://registry.yarnpkg.com/immutable/-/immutable-4.0.0-rc.9.tgz#1e6e0094e649013ec3742d2b5aeeca5eeda4f0bf"
+
 imurmurhash@^0.1.4:
   version "0.1.4"
   resolved "https://registry.yarnpkg.com/imurmurhash/-/imurmurhash-0.1.4.tgz#9218b9b2b928a238b13dc4fb6b6d576f231453ea"