dart:platform | Remove isWindows, isLinux, isAndroid, isMacOS from dart:platform.

runtime/lib/platform_patch.dart

Index: runtime/lib/platform_patch.dart
diff --git a/runtime/lib/platform_patch.dart b/runtime/lib/platform_patch.dart
index cc1c1879fa362ec2fff4fca3d1d1443dd1ea1d5e..80dfef91e3dd7aac483706dde9fc08f125139fce 100644
--- a/runtime/lib/platform_patch.dart
+++ b/runtime/lib/platform_patch.dart
@@ -22,14 +22,6 @@ patch List<String> get executableArguments => _platform.executableArguments;
 
 patch String get packageRoot => _platform.packageRoot;
 
-patch bool get isLinux => _platform.operatingSystem == "linux";
-
-patch bool get isMacOS => _platform.operatingSystem == "macos";
-
-patch bool get isWindows => _platform.operatingSystem == "windows";
-
-patch bool get isAndroid => _platform.operatingSystem == "android";
-
 class _Platform {
   int get numberOfProcessors;
   String get pathSeparator;
@@ -52,7 +44,7 @@ class _DefaultPlatform implements _Platform {
   }
 
   String get pathSeparator {
-    return "/";
+    return null;
   }
 
   String get operatingSystem {