[connectivity_plus] eliminate indirect FFI dependency (#65)

packages/connectivity_plus_linux/lib/connectivity_plus_linux.dart

@@ -1,4 +1,8 @@
 /// The Linux implementation of `connectivity_plus`.
 library connectivity_plus_linux;
 
-export 'src/connectivity.dart';
+// connectivity_plus_linux depends on dbus which uses FFI internally; export
+// a stub for platforms that don't support FFI (e.g., web) to avoid having
+// transitive dependencies break web compilation.
+export 'src/connectivity_stub.dart'
+    if (dart.library.ffi) 'src/connectivity_real.dart';

packages/connectivity_plus_linux/lib/src/connectivity_stub.dart

@@ -0,0 +1,16 @@
+import 'package:connectivity_plus_platform_interface/connectivity_plus_platform_interface.dart';
+
+/// A stub implementation to satisfy compilation of multi-platform packages that
+/// depend on connectivity_plus_linux. This should never actually be created.
+///
+/// Notably, because connectivity_plus needs to manually register
+/// connectivity_plus_linux, anything with a transitive dependency on
+/// connectivity_plus will also depend on connectivity_plus_linux, not just at
+/// the pubspec level but the code level.
+class ConnectivityLinux extends ConnectivityPlatform {
+  /// Errors on attempted instantiation of the stub. It exists only to satisfy
+  /// compile-time dependencies, and should never actually be created.
+  ConnectivityLinux() {
+    assert(false);
+  }
+}

packages/connectivity_plus_linux/test/connectivity_plus_linux_test.dart

@@ -1,4 +1,4 @@
-import 'package:connectivity_plus_linux/src/connectivity.dart';
+import 'package:connectivity_plus_linux/src/connectivity_real.dart';
 import 'package:connectivity_plus_linux/src/network_manager.dart';
 import 'package:connectivity_plus_platform_interface/connectivity_plus_platform_interface.dart';
 import 'package:flutter_test/flutter_test.dart';