move static fields to PlatformBehaviors

lib/terminal/platform.dart

@@ -2,7 +2,9 @@ class PlatformBehavior {
   const PlatformBehavior({this.oscTerminators});
 
   final Set<int> oscTerminators;
+}
 
+class PlatformBehaviors {
   static const unix = PlatformBehavior(oscTerminators: {0x07, 0x5c});
   static const windows = PlatformBehavior(oscTerminators: {0x07, 0x00});
 }

lib/terminal/terminal.dart

@@ -31,7 +31,7 @@ class Terminal with Observable {
     this.onBell,
     this.onTitleChange,
     this.onIconChange,
-    this.platform = PlatformBehavior.unix,
+    this.platform = PlatformBehaviors.unix,
     this.theme = TerminalThemes.defaultTheme,
     int maxLines,
   }) {