[infra] Use set -x in flutter scripts

Change-Id: I3f938b4279aee7d4cd19e08e7c90c3ddba570e5b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/192747
Reviewed-by: Jonas Jensen <jonasfj@google.com>

tools/bots/flutter/analyze_flutter_engine.sh

@@ -5,7 +5,7 @@
 
 # Analyze Dart code in the flutter/engine repo.
 
-set -e
+set -ex
 
 checkout=$(pwd)
 dart=$checkout/out/ReleaseX64/dart-sdk/bin/dart

tools/bots/flutter/analyze_flutter_flutter.sh

@@ -5,7 +5,7 @@
 
 # Analyze Dart code in the flutter/flutter repo.
 
-set -e
+set -ex
 
 checkout=$(pwd)
 dart=$checkout/out/ReleaseX64/dart-sdk/bin/dart

tools/bots/flutter/compile_flutter.sh

@@ -5,7 +5,7 @@
 
 # Compile flutter tests with a locally built SDK.
 
-set -e
+set -ex
 
 prepareOnly=false
 leakTest=false