Snapshot the felt tool for faster start-up

lib/web_ui/dev/felt

@@ -30,6 +30,11 @@ OUT_DIR="${ENGINE_SRC_DIR}/out"
 HOST_DEBUG_UNOPT_DIR="${ENGINE_SRC_DIR}/out/host_debug_unopt"
 DART_SDK_DIR="${ENGINE_SRC_DIR}/out/host_debug_unopt/dart-sdk"
 GN="${FLUTTER_DIR}/tools/gn"
+DART_TOOL_DIR="${WEB_UI_DIR}/.dart_tool"
+SNAPSHOT_PATH="${DART_TOOL_DIR}/felt.snapshot"
+STAMP_PATH="${DART_TOOL_DIR}/felt.snapshot.stamp"
+SCRIPT_PATH="${DEV_DIR}/felt.dart"
+REVISION="$(cd "$FLUTTER_DIR"; git rev-parse HEAD)"
 
 if [ ! -d "${OUT_DIR}" ] || [ ! -d "${HOST_DEBUG_UNOPT_DIR}" ]
 then
@@ -39,10 +44,24 @@ then
   ninja -C $HOST_DEBUG_UNOPT_DIR
 fi
 
-echo "Running \`pub get\` in 'engine/src/flutter/lib/web_ui'"
-(cd "$WEB_UI_DIR"; $DART_SDK_DIR/bin/pub get)
+# Invalidate cache if:
+#  * SNAPSHOT_PATH is not a file, or
+#  * STAMP_PATH is not a file with nonzero size, or
+#  * Contents of STAMP_PATH is not our local git HEAD revision, or
+#  * pubspec.yaml last modified after pubspec.lock
+#
+# To force invalidate the cache run `felt clean` or `rm .dart_tool/felt.shapshot.stamp`
+if [[ ! -f $SNAPSHOT_PATH || ! -s "$STAMP_PATH" || "$(cat "$STAMP_PATH")" != "$REVISION" || "$WEB_UI_DIR/pubspec.yaml" -nt "$WEB_UI_DIR/pubspec.lock" ]]; then
+  echo "Snapshotting the felt tool for faster subsequent runs."
+  echo "Running \`pub get\` in 'engine/src/flutter/lib/web_ui'"
+  (cd "$WEB_UI_DIR"; $DART_SDK_DIR/bin/pub get)
 
-echo "Running \`pub get\` in 'engine/src/flutter/web_sdk/web_engine_tester'"
-(cd "$FLUTTER_DIR/web_sdk/web_engine_tester"; $DART_SDK_DIR/bin/pub get)
+  echo "Running \`pub get\` in 'engine/src/flutter/web_sdk/web_engine_tester'"
+  (cd "$FLUTTER_DIR/web_sdk/web_engine_tester"; $DART_SDK_DIR/bin/pub get)
+  mkdir -p $DART_TOOL_DIR
 
-$DART_SDK_DIR/bin/dart "$DEV_DIR/felt.dart" $@
+  "$DART_SDK_DIR/bin/dart" --snapshot="$SNAPSHOT_PATH" --packages="$WEB_UI_DIR/.packages" "$SCRIPT_PATH"
+  echo "$REVISION" > "$STAMP_PATH"
+fi
+
+$DART_SDK_DIR/bin/dart --packages="$WEB_UI_DIR/.packages" "$SNAPSHOT_PATH" $@