Ensure that Dart SDK dirs have appropriate permissions (#26315)

This is a partial re-application of #24669

https://github.com/flutter/flutter/issues/24413

bin/internal/update_dart_sdk.sh

@@ -44,9 +44,11 @@ if [ ! -f "$ENGINE_STAMP" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP"` ]; t
   case "$(uname -s)" in
     Darwin)
       DART_ZIP_NAME="dart-sdk-darwin-x64.zip"
+      IS_USER_EXECUTABLE="-perm +100"
       ;;
     Linux)
       DART_ZIP_NAME="dart-sdk-linux-x64.zip"
+      IS_USER_EXECUTABLE="-perm /u+x"
       ;;
     *)
       echo "Unknown operating system. Cannot install Dart SDK."
@@ -65,7 +67,7 @@ if [ ! -f "$ENGINE_STAMP" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP"` ]; t
 
   # install the new sdk
   rm -rf -- "$DART_SDK_PATH"
-  mkdir -p -- "$DART_SDK_PATH"
+  mkdir -m 755 -p -- "$DART_SDK_PATH"
   DART_SDK_ZIP="$FLUTTER_ROOT/bin/cache/$DART_ZIP_NAME"
 
   curl --continue-at - --location --output "$DART_SDK_ZIP" "$DART_SDK_URL" 2>&1 || {
@@ -87,6 +89,8 @@ if [ ! -f "$ENGINE_STAMP" ] || [ "$ENGINE_VERSION" != `cat "$ENGINE_STAMP"` ]; t
     exit 1
   }
   rm -f -- "$DART_SDK_ZIP"
+  find "$DART_SDK_PATH" -type d -exec chmod 755 {} \;
+  find "$DART_SDK_PATH" -type f $IS_USER_EXECUTABLE -exec chmod a+x,a+r {} \;
   echo "$ENGINE_VERSION" > "$ENGINE_STAMP"
 
   # delete any temporary sdk path