[camerax] Ignore new unreachable_switch_default warning.

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.

In the vast majority of cases, the most reasonable way to address the
warning is to remove the unreachable `default` clause. However, in a
few rare cases, it makes sense to keep the `default` clause, because
it's intentionally future-proofing the code in case new possible
values are added to the enum being switched on.

Three of these rare cases crop up in the camerax package. This change
adds `ignore` comments to avoid a spurious warning.

packages/camera/camera_android_camerax/lib/src/capture_request_options.dart

@@ -115,7 +115,7 @@ class _CaptureRequestOptionsHostApiImpl extends CaptureRequestOptionsHostApi {
         // This ignore statement is safe beause this error will be useful when
         // a new CaptureRequestKeySupportedType is being added, but the logic in
         // this method has not yet been updated.
-        // ignore: no_default_cases
+        // ignore: no_default_cases, unreachable_switch_default
         default:
           throw ArgumentError(CaptureRequestOptions
               .getUnsupportedCaptureRequestKeyTypeErrorMessage(key));

packages/camera/camera_android_camerax/lib/src/live_data.dart

@@ -185,7 +185,7 @@ class LiveDataFlutterApiImpl implements LiveDataFlutterApi {
       // This ignore statement is safe beause this error will be useful when
       // a new LiveDataSupportedType is being added, but the logic in this method
       // has not yet been updated.
-      // ignore: no_default_cases
+      // ignore: no_default_cases, unreachable_switch_default
       default:
         throw ArgumentError(LiveData.unsupportedLiveDataTypeErrorMessage);
     }

packages/camera/camera_android_camerax/test/capture_request_options_test.dart

@@ -132,7 +132,7 @@ void main() {
           // This ignore statement is safe beause this will test when
           // a new CaptureRequestKeySupportedType is being added, but the logic in
           // in the CaptureRequestOptions class has not yet been updated.
-          // ignore: no_default_cases
+          // ignore: no_default_cases, unreachable_switch_default
           default:
             fail(
                 'Option $option contains unrecognized CaptureRequestKeySupportedType key ${option.$1}');