Enforce lockfile is kept up to date (facebook#21739)

Summary:
Adds a new script that runs Yarn with --frozen-lockfile, and fails if Yarn finds the lockfile is out of sync.

Keeping the lockfile in sync with package.json will ensure our internal open source tests can run offline, as our offline mirror is updated whenever the lockfile changes.

Reverted yarn.lock to https://raw.githubusercontent.com/facebook/react-native/6eeff75849c9b8bf91592c1b7906b4dab8fba518/yarn.lock, then ran `scripts/circleci/validate_yarn_lockfile.sh`, and verified the script failed as expected.

I then ran Yarn without the --frozen-lockfile flag, and reran the validation script. This time, the script finished successfully.
Pull Request resolved: facebook#21739

Differential Revision: D10365642

Pulled By: hramos

fbshipit-source-id: 9ab7f03919427a86b12901d4c422ef04dd0839f6

.circleci/config.yml

@@ -198,6 +198,7 @@ aliases:
     command: |
       ./scripts/circleci/check_license.sh
       ./scripts/circleci/check_cache.sh
+      ./scripts/circleci/validate_yarn_lockfile.sh
     when: always
 
   - &download-dependencies-gradle

scripts/circleci/validate_yarn_lockfile.sh

@@ -0,0 +1,39 @@
+#!/bin/bash
+
+# Abort the mission if any command fails
+set -e
+
+# Allow the script to be invoked from various environments
+if [[ -z "${OVERRIDE_YARN_BINARY}" ]]; then
+  YARN_BINARY=$(command -v yarn)
+else
+  YARN_BINARY="${OVERRIDE_YARN_BINARY}"
+fi
+
+REACT_NATIVE_TEMP_DIR=$(mktemp -d /tmp/react-native-XXXXXXXX)
+
+function cleanup {
+  set +e
+  rm -rf "$REACT_NATIVE_TEMP_DIR"
+  set -e
+}
+
+function msg {
+  echo -e " "
+  echo -e "\\x1B[36m${1}\\x1B[0m";
+  echo -e "\\x1B[36m${1//?/=}\\x1B[0m"
+}
+
+trap cleanup EXIT
+
+cp -R ./package.json "$REACT_NATIVE_TEMP_DIR"
+cp -R ./yarn.lock "$REACT_NATIVE_TEMP_DIR"
+pushd "$REACT_NATIVE_TEMP_DIR" >/dev/null
+
+if ! $YARN_BINARY --ignore-scripts --silent --non-interactive --mutex network --frozen-lockfile; then
+  msg "Yarn validation failed."
+  echo "This means the package.json and yarn.lock disagree in some way."
+  echo "Try fixing it by running \`yarn\` and committing the changes."
+fi
+
+popd >/dev/null