[connectivity_platform_interface] Add ConnectivityResult.unknown.

Some platforms might not be able to determine the connectivity status of
the device on which the app is running (like some desktop Web browsers).

This allows users of the `connectivity` plugin to distinguish between
"no connectivity" and "connectivity couldn't be determined".

This requires a Major Version bump for users of the plugin who may be
switch/case on ConnectivityResult values, since Dart forces users to be
exhaustive in those cases (if they don't have a "default" entry, this
new value becomes a compilation error in their code).

This will also cause a Major Version bump in the core `connectivity`
plugin itself.

packages/connectivity/connectivity_platform_interface/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 2.0.0
+
+* Added `ConnectivityResult.unknown`, for the cases where the plugin is unable to determine the
+connectivity status of the device. _(This happens mostly in the `web` platform.)_
+
 ## 1.0.2
 
 * Bring ConnectivityResult and LocationAuthorizationStatus enums from the core package.

packages/connectivity/connectivity_platform_interface/lib/src/enums.dart

@@ -7,7 +7,10 @@ enum ConnectivityResult {
   mobile,
 
   /// None: Device not connected to any network
-  none
+  none,
+
+  /// Unknown: The plugin wasn't able to determine the connectivity status of the device
+  unknown,
 }
 
 /// The status of the location service authorization.

packages/connectivity/connectivity_platform_interface/pubspec.yaml

@@ -3,7 +3,7 @@ description: A common platform interface for the connectivity plugin.
 homepage: https://github.com/flutter/plugins/tree/master/packages/connectivity/connectivity_platform_interface
 # NOTE: We strongly prefer non-breaking changes, even at the expense of a
 # less-clean API. See https://flutter.dev/go/platform-interface-breaking-changes
-version: 1.0.2
+version: 2.0.0
 
 dependencies:
   flutter: