[Danger] Minor fixes (#12606)

* Don't download bundle stats from master on CI

This was temporarily necessary in the past because we didn't have the logic that downloads actual *merge base* stats.

We do have that now as part of the Danger script. So we can remove this.

* Use absolute threshold for whether to show a change

* Download master stats, but only for other master builds

* Rewrite sizes

dangerfile.js

@@ -117,14 +117,12 @@ function git(args) {
     previousBuildResults
   );
 
-  const percentToWarrentShowing = 1;
   const packagesToShow = results
     .filter(
       r =>
-        Math.abs(r.prevFileSizeChange) >= percentToWarrentShowing ||
-        Math.abs(r.prevGzipSizeChange) >= percentToWarrentShowing
+        Math.abs(r.prevFileSizeAbsoluteChange) >= 300 || // bytes
+        Math.abs(r.prevGzipSizeAbsoluteChange) >= 100 // bytes
     )
-
     .map(r => r.packageName);
 
   if (packagesToShow.length) {

scripts/circleci/build.sh

@@ -2,10 +2,13 @@
 
 set -e		
 
-# Update the local size measurements to the master version
-# so that the size diff printed at the end of the build is
-# accurate.
-curl -o scripts/rollup/results.json http://react.zpao.com/builds/master/latest/results.json
+# On master, download the bundle sizes from last master build so that
+# the size printed in the CI logs for master commits is accurate.
+# We don't do it for pull requests because those are compared against
+# the merge base by Dangerfile instead. See https://github.com/facebook/react/pull/12606.
+if [ -z "$CI_PULL_REQUEST" ]; then
+  curl -o scripts/rollup/results.json http://react.zpao.com/builds/master/latest/results.json
+fi
 
 yarn build --extract-errors
 # Note: since we run the full build including extracting error codes,