[dwds] Delete unreachable default clauses. (#2484)

The Dart analyzer will soon be changed so that if the `default` clause
of a `switch` statement is determined to be unreachable by the
exhaustiveness checker, a new warning of type
`unreachable_switch_default` will be issued. This parallels the
behavior of the existing `unreachable_switch_case` warning, which is
issued whenever a `case` clause of a `switch` statement is determined
to be unreachable. For details see
dart-lang/sdk#54575.

This PR deletes unreachable `default` clauses from `webdev/dwds` now,
to avoid a spurious warning when the analyzer change lands.

dwds/lib/src/debugging/instance.dart

@@ -170,7 +170,6 @@ class InstanceHelper extends Domain {
       case RuntimeObjectKind.object:
       case RuntimeObjectKind.nativeError:
       case RuntimeObjectKind.nativeObject:
-      default:
         return await _plainInstanceFor(
           metaData,
           remoteObject,

dwds/lib/src/servers/extension_debugger.dart

@@ -364,8 +364,6 @@ class ExtensionDebugger implements RemoteDebugger {
         return 'none';
       case PauseState.uncaught:
         return 'uncaught';
-      default:
-        throw ArgumentError('unknown state: $state');
     }
   }