chore(travis): fail jobs that have a mismatched pubspec.lock

We expect that, for a clean checkout, running "pub get" will not update
the pubspec.lock file.  Any such updates should have been made along
with the corresponding change to pubspec.yaml or must be part of the
commit being tested.  Otherwise, that specific job should fail so the
author can resubmit with the updated pubspec.lock file.

Author: chirayuk

scripts/travis/setup.sh

@@ -24,7 +24,22 @@ fi
 echo =============================================================================
 . ./scripts/env.sh
 $DART --version
+PUBSPEC_HASH=$(shasum pubspec.lock)
 $PUB install
+if [[ $(shasum pubspec.lock) != "$PUBSPEC_HASH" ]]; then
+  if [ "$USE_G3" = "YES" ]; then
+    echo ERROR: This commit causes pubspec.lock for g3v1x to change.  g3v1x
+    echo uses dependency overrides to tests against specific versions of packages.
+    echo Please ensure that these updated packages are available for g3v1x, update
+    echo the dependency overrides and the pubspec.lock for g3v1x and push that.  You
+    echo may test this job again after that.  Until then, this is considered a
+    echo failure for the USE_G3=YES jobs.
+  else
+    echo ERROR: This commit causes pubspec.lock to change.  Please include the
+    echo pubspec.lock changes in this commit and re-run this test.
+  fi
+  exit 1
+fi
 
 # Record dart version for tests.
 echo $'import "dart:js";\n\nmain() {\n  context["DART_VERSION"] = \''"$($DART --version 2>&1)"$'\';\n}' > test/dart_version.dart