Add devDeps for flow and postcss

  - Add `flow-bin` to enable manual flow checking.  The version added
    is the last version used by the project.  To run flow, use the
    command:

      yarn flow check

  - Explicitly add `postcss` to remove the `yarn install` warning about
    missing peer dependencies.  This version was already referenced in
    yarn.lock so no new dependencies have been added.

Author: sjd78

package.json

@@ -35,6 +35,7 @@
     "eslint-webpack-plugin": "2.5.4",
     "file-loader": "6.0.0",
     "filesize": "3.6.1",
+    "flow-bin": "0.123.0",
     "fs-extra": "0.30.0",
     "gzip-size": "3.0.0",
     "handlebars": "4.7.7",
@@ -49,6 +50,7 @@
     "opn": "4.0.2",
     "optimize-css-assets-webpack-plugin": "6.0.0",
     "path-exists": "3.0.0",
+    "postcss": "7.0.35",
     "postcss-loader": "4.2.0",
     "postcss-preset-env": "6.7.0",
     "postcss-safe-parser": "5.0.2",

yarn.lock

@@ -6446,6 +6446,11 @@ flatten@^1.0.2:
   resolved "https://registry.yarnpkg.com/flatten/-/flatten-1.0.3.tgz#c1283ac9f27b368abc1e36d1ff7b04501a30356b"
   integrity sha512-dVsPA/UwQ8+2uoFe5GHtiBMu48dWLTdsuEd7CKGlZlD78r1TTWBvDuFaFGKCo/ZfEr95Uk56vZoX86OsHkUeIg==
 
+[email protected]:
+  version "0.123.0"
+  resolved "https://registry.yarnpkg.com/flow-bin/-/flow-bin-0.123.0.tgz#7ba61a0b8775928cf4943ccf78eed2b1b05f7b3a"
+  integrity sha512-Ylcf8YDIM/KrqtxkPuq+f8O+6sdYA2Nuz5f+sWHlp539DatZz3YMcsO1EiXaf1C11HJgpT/3YGYe7xZ9/UZmvQ==
+
 flush-write-stream@^1.0.0:
   version "1.1.1"
   resolved "https://registry.yarnpkg.com/flush-write-stream/-/flush-write-stream-1.1.1.tgz#8dd7d873a1babc207d94ead0c2e0e44276ebf2e8"
@@ -11880,7 +11885,7 @@ postcss-values-parser@^2.0.0, postcss-values-parser@^2.0.1:
     indexes-of "^1.0.1"
     uniq "^1.0.1"
 
-postcss@^7.0.14, postcss@^7.0.17, postcss@^7.0.2, postcss@^7.0.32, postcss@^7.0.5, postcss@^7.0.6:
+postcss@7.0.35, postcss@^7.0.14, postcss@^7.0.17, postcss@^7.0.2, postcss@^7.0.32, postcss@^7.0.5, postcss@^7.0.6:
   version "7.0.35"
   resolved "https://registry.yarnpkg.com/postcss/-/postcss-7.0.35.tgz#d2be00b998f7f211d8a276974079f2e92b970e24"
   integrity sha512-3QT8bBJeX/S5zKTTjTCIjRF3If4avAT6kqxcASlTWEtAFCb9NH0OUxNDfgZSWdP5fJnBYCMEWkIFfWeugjzYMg==