Update core-js

Update our JS standard library polyfills to the latest stable release.

package.json

@@ -29,7 +29,7 @@
     "codecov": "^1.0.1",
     "coffeeify": "^1.0.0",
     "commander": "^2.9.0",
-    "core-js": "^1.2.5",
+    "core-js": "^2.5.7",
     "cross-env": "^5.1.4",
     "custom-event": "^1.0.1",
     "diff": "^2.2.2",

yarn.lock

@@ -1663,14 +1663,14 @@ copy-descriptor@^0.1.0:
   version "0.1.1"
   resolved "https://registry.yarnpkg.com/copy-descriptor/-/copy-descriptor-0.1.1.tgz#676f6eb3c39997c2ee1ac3a924fd6124748f578d"
 
-core-js@^1.2.5:
-  version "1.2.7"
-  resolved "https://registry.yarnpkg.com/core-js/-/core-js-1.2.7.tgz#652294c14651db28fa93bd2d5ff2983a4f08c636"
-
 core-js@^2.2.0, core-js@^2.4.0, core-js@^2.5.0:
   version "2.5.5"
   resolved "https://registry.yarnpkg.com/core-js/-/core-js-2.5.5.tgz#b14dde936c640c0579a6b50cabcc132dd6127e3b"
 
+core-js@^2.5.7:
+  version "2.5.7"
+  resolved "https://registry.yarnpkg.com/core-js/-/core-js-2.5.7.tgz#f972608ff0cead68b841a16a932d0b183791814e"
+
 core-util-is@1.0.2, core-util-is@~1.0.0:
   version "1.0.2"
   resolved "https://registry.yarnpkg.com/core-util-is/-/core-util-is-1.0.2.tgz#b5fd54220aa2bc5ab57aab7140c940754503c1a7"