Fix docs.sh to be more idomatic Bash, runnable from any dir (#19922)

dev/bots/docs.sh

 #!/bin/bash
 set -e
 
-echo "Running docs.sh"
+function script_location() {
+  local script_location="${BASH_SOURCE[0]}"
+  # Resolve symlinks
+  while [[ -h "$script_location" ]]; do
+    DIR="$(cd -P "$( dirname "$script_location")" >/dev/null && pwd)"
+    script_location="$(readlink "$script_location")"
+    [[ "$script_location" != /* ]] && script_location="$DIR/$script_location"
+  done
+  echo "$(cd -P "$(dirname "$script_location")" >/dev/null && pwd)"
+}
 
-# If you want to run this script locally, make sure you run it from
-# the root of the flutter repository.
-export FLUTTER_ROOT="$PWD"
-export PATH="$PWD/bin:$PATH"
+# So that users can run this script from anywhere and it will work as expected.
+SCRIPT_LOCATION="$(script_location)"
+FLUTTER_ROOT="$(dirname "$(dirname "$SCRIPT_LOCATION")")"
+
+if [[ ! -d "$FLUTTER_ROOT" || ! -f "$FLUTTER_ROOT/bin/flutter" ]]; then
+  echo "Unable to locate the Flutter installation (using FLUTTER_ROOT: $FLUTTER_ROOT)"
+  exit 1
+fi
+
+FLUTTER_BIN="$FLUTTER_ROOT/bin"
+DART_BIN="$FLUTTER_ROOT/bin/cache/dart-sdk/bin"
+FLUTTER="$FLUTTER_BIN/flutter"
+DART="$DART_BIN/dart"
+PUB="$DART_BIN/pub"
+export PATH="$FLUTTER_BIN:$DART_BIN:$PATH"
 
 # This is called from travis_upload.sh on Travis.
 
-# Make sure dart is installed
-bin/flutter --version
+# Make sure dart is installed by invoking flutter to download it.
+"$FLUTTER" --version
 
 # If the pub cache directory exists in the root, then use that.
 FLUTTER_PUB_CACHE="$FLUTTER_ROOT/.pub-cache"
-if [ -d "$FLUTTER_PUB_CACHE" ]; then
+if [[ -d "$FLUTTER_PUB_CACHE" ]]; then
   # This has to be exported, because pub interprets setting it
   # to the empty string in the same way as setting it to ".".
   export PUB_CACHE="${PUB_CACHE:-"$FLUTTER_PUB_CACHE"}"
 fi
 
 # Install dartdoc.
-bin/cache/dart-sdk/bin/pub global activate dartdoc 0.20.2
+"$PUB" global activate dartdoc 0.20.2
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.
-(cd dev/tools; ../../bin/cache/dart-sdk/bin/pub get)
-bin/cache/dart-sdk/bin/dart dev/tools/dartdoc.dart
-bin/cache/dart-sdk/bin/dart dev/tools/java_and_objc_doc.dart
+(cd "$FLUTTER_ROOT/dev/tools" && "$PUB" get)
+(cd "$FLUTTER_ROOT" && "$DART" "$FLUTTER_ROOT/dev/tools/dartdoc.dart")
+(cd "$FLUTTER_ROOT" && "$DART" "$FLUTTER_ROOT/dev/tools/java_and_objc_doc.dart")
 
 # Ensure google webmaster tools can verify our site.
-cp dev/docs/google2ed1af765c529f57.html dev/docs/doc
-
-# Upload new API docs when on Travis
-if [ "$TRAVIS_PULL_REQUEST" == "false" ]; then
-  echo "This is not a pull request; considering whether to upload docs... (branch=$TRAVIS_BRANCH)"
-  if [ "$TRAVIS_BRANCH" == "master" -o "$TRAVIS_BRANCH" == "beta" ]; then
-    cd dev/docs
+cp "$FLUTTER_ROOT/dev/docs/google2ed1af765c529f57.html" "$FLUTTER_ROOT/dev/docs/doc"
 
-    if [ "$TRAVIS_BRANCH" == "master" ]; then
+# Upload new API docs when on Cirrus
+if [[ -n "$CIRRUS_CI" && -z "$CIRRUS_PR" ]]; then
+  echo "This is not a pull request; considering whether to upload docs... (branch=$CIRRUS_BRANCH)"
+  if [[ "$CIRRUS_BRANCH" == "master" || "$CIRRUS_BRANCH" == "beta" ]]; then
+    if [[ "$CIRRUS_BRANCH" == "master" ]]; then
       echo "Updating master docs: https://master-docs-flutter-io.firebaseapp.com/"
-      echo -e "User-agent: *\nDisallow: /" > doc/robots.txt
-      while : ; do
-        firebase deploy --project master-docs-flutter-io && break
+      echo -e "User-agent: *\nDisallow: /" > "$FLUTTER_ROOT/dev/docs/doc/robots.txt"
+      while true; do
+        (cd "$FLUTTER_ROOT/dev/docs" && firebase deploy --project master-docs-flutter-io) && break
         echo Error: Unable to deploy documentation to firebase. Retrying in five seconds...
         sleep 5
       done
     fi
 
-    if [ "$TRAVIS_BRANCH" == "beta" ]; then
+    if [[ "$CIRRUS_BRANCH" == "beta" ]]; then
       echo "Updating beta docs: https://docs.flutter.io/"
-      while : ; do
-        firebase deploy --project docs-flutter-io && break
+      while true; do
+        (cd "$FLUTTER_ROOT/dev/docs" && firebase deploy --project docs-flutter-io) && break
         echo Error: Unable to deploy documentation to firebase. Retrying in five seconds...
         sleep 5
       done