Update sizebot to match name of CircleCI build job (#15714)

The sizebot scrapes the GitHub `/statuses` endpoint to get the lastest
CircleCI build number for master, in order to fetch the bundle size
info for that build, which are stored as build artifacts. (There's gotta
be a better way to do this, but that's what we have for now.) This
updates the script to match the name of the updated CircleCI job that
generates the bundle sizes.

.circleci/config.yml

@@ -126,7 +126,6 @@ jobs:
       - run: ./scripts/circleci/update_package_versions.sh
       - run: ./scripts/circleci/build.sh
       - run: cp ./scripts/rollup/results.json ./build/bundle-sizes.json
-      - run: node ./scripts/tasks/danger
       - run: ./scripts/circleci/upload_build.sh
       - run: ./scripts/circleci/pack_and_store_artifact.sh
       - store_artifacts:
@@ -143,6 +142,18 @@ jobs:
             - facebook-www
             - node_modules
             - react-native
+            - bundle-sizes.json
+
+  sizebot:
+    build:
+    docker: *docker
+    environment: *environment
+    steps:
+      - checkout
+      - attach_workspace: *attach_workspace
+      - *restore_yarn_cache
+      - *run_yarn
+      - run: node ./scripts/tasks/danger
 
   test_build:
     docker: *docker
@@ -193,6 +204,9 @@ workflows:
       - build:
           requires:
             - setup
+      - sizebot:
+          requires:
+            - build
       - test_build:
           requires:
             - build

dangerfile.js

@@ -36,6 +36,10 @@ if (!existsSync('./build/bundle-sizes.json')) {
   // This indicates the build failed previously.
   // In that case, there's nothing for the Dangerfile to do.
   // Exit early to avoid leaving a redundant (and potentially confusing) PR comment.
+  warn(
+    'No bundle size information found. This indicates the build ' +
+      'job failed.'
+  );
   process.exit(0);
 }
 
@@ -127,7 +131,8 @@ function git(args) {
     const statuses = await statusesResponse.json();
     for (let i = 0; i < statuses.length; i++) {
       const status = statuses[i];
-      if (status.context === 'ci/circleci') {
+      // This must match the name of the CI job that creates the build artifacts
+      if (status.context === 'ci/circleci: build') {
         if (status.state === 'success') {
           baseCIBuildId = /\/facebook\/react\/([0-9]+)/.exec(
             status.target_url
@@ -271,5 +276,7 @@ function git(args) {
   </details>
   `;
     markdown(summary);
+  } else {
+    markdown('No significant bundle size changes to report.');
   }
 })();