Replace some Map.containsKey with patterns (#490)

In a few expressions where we check for the presence of a key, and then
immediately read that key, refactor to an if-case which decomposes the
variable inline.

mono_repo/lib/src/package_config.dart

@@ -95,9 +95,7 @@ class PackageConfig {
       final stageYaml = stage.items;
       for (var job in stageYaml) {
         var jobSdks = rawConfig.sdks;
-        if (job is Map && job.containsKey('sdk')) {
-          final jobValue = job['sdk'];
-
+        if (job case {'sdk': final jobValue}) {
           jobSdks = (jobValue is List)
               ? jobSdks = List.from(jobValue)
               : [jobValue as String];
@@ -149,8 +147,7 @@ class PackageConfig {
         }
 
         var jobOses = rawConfig.oses;
-        if (job is Map && job.containsKey('os')) {
-          final jobValue = job['os'];
+        if (job case {'os': final jobValue}) {
           if (jobValue is List) {
             jobOses = jobValue.cast<String>();
           } else {

mono_repo/lib/src/root_config.dart

@@ -160,9 +160,8 @@ class RootConfig extends ListBase<PackageConfig> {
       } else if (yaml is Map) {
         const usesKey = 'uses';
 
-        if (yaml.containsKey(usesKey)) {
+        if (yaml case {usesKey: final usage as String}) {
           // dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
-          final usage = yaml[usesKey] as String;
           final match = usageRegex.firstMatch(usage);
           if (match != null) {
             result['${match.group(1)}/${match.group(2)}'] = match.group(3)!;