Update Flutter API docs continuously

The patch teaches Travis how to update our API docs continuously as we land
patches in master.

travis/setup.sh

 #!/bin/bash
-set -ex
+set -e
+
+if [ $TRAVIS_PULL_REQUEST = "false" ]; then
+  echo $KEY_FILE | base64 --decode > gcloud_key_file.json
+fi
+
+set -x
 
 dart dev/update_packages.dart
 (cd packages/unit; ../../bin/flutter cache populate)
+
+if [ $TRAVIS_PULL_REQUEST = "false" ]; then
+  export CLOUDSDK_CORE_DISABLE_PROMPTS=1
+  curl https://sdk.cloud.google.com | bash
+fi

travis/test.sh

@@ -14,3 +14,14 @@ set -ex
 (cd packages/newton; pub run test -j1)
 # (cd packages/playfair; ) # No tests to run.
 # (cd packages/updater; ) # No tests to run.
+
+if [ $TRAVIS_PULL_REQUEST = "false" ]; then
+  if [ $TRAVIS_BRANCH = "master"]; then
+    (cd packages/flutter; dartdoc)
+
+    GSUTIL=$HOME/google-cloud-sdk/bin/gsutil
+    GCLOUD=$HOME/google-cloud-sdk/bin/gcloud
+    $GCLOUD auth activate-service-account --key-file gcloud_key_file.json
+    $GSUTIL -m rsync -r -d packages/flutter/doc/api gs://docs.domokit.org/flutter
+  fi
+fi