Unable to clear polylines set for map (worked previously)

**Describe the bug**
Calling the clear method on map.polylines throws "Unhandled Exception: Unsupported operation: Cannot change an unmodifiable set"

**To Reproduce**
Steps to reproduce the behavior:
1. Just add a clear() call somewhere, though may need to add a few polylines if you don't have them in the sample.

**Expected behavior**
the set should be mutable and the clear should remove all displayed polylines.

**Screenshots**
n/a.

**Smartphone (please complete the following information):**
- Device: Iphone simulator (any)
- Version [e.g. 22]

**flutter doctor**
/Volumes/devstuff/Developer/flutter/bin/flutter doctor --verbose
[✓] Flutter (Channel stable, 2.5.1, on macOS 11.6 20G165 darwin-x64, locale en-US)
• Flutter version 2.5.1 at /Volumes/devstuff/Developer/flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision ffb2ecea52 (7 weeks ago), 2021-09-17 15:26:33 -0400
• Engine revision b3af521a05
• Dart version 2.14.2

[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
• Android SDK at /Volumes/devstuff/Android/sdk/
• Platform android-29, build-tools 29.0.3
• Java binary at: /Library/Java/JavaVirtualMachines/jdk-13.0.1.jdk/Contents/Home/bin/java
• Java version Java(TM) SE Runtime Environment (build 13.0.1+9)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS
• Xcode at /Applications/Xcode.app/Contents/Developer
• Xcode 13.1, Build version 13A1030d
• CocoaPods version 1.10.0

[✓] Chrome - develop for the web
• Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2020.3)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/6351-dart
• Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)

[✓] Android Studio (version 2020.3)
• Android Studio at /Volumes/Macintosh HD/Applications/Android Studio.app/Contents
• Flutter plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/6351-dart
• Java version OpenJDK Runtime Environment (build 11.0.10+0-b96-7281165)

[✓] VS Code (version 1.60.0)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.20.0

[✓] Connected device (3 available)
• iPhone 8 (mobile) • B094B1C2-EBDB-404C-A35A-58F6931095AC • ios • com.apple.CoreSimulator.SimRuntime.iOS-15-0 (simulator)
• iPhone 13 Pro (mobile) • 38883115-5DA6-4341-A3DE-5C06313A80B0 • ios • com.apple.CoreSimulator.SimRuntime.iOS-15-0 (simulator)
• Chrome (web) • chrome • web-javascript • Google Chrome 95.0.4638.69

• No issues found!