List output with differing types should be more resilient

kubectl get can output a series of objects as a List in versioned
form, but not all API objects are available in the same schema.
Make the act of converting a []runtime.Object to api.List more
robust and add a test to verify its behavior in Get.

Makes it easier for client code to output unified objects.

pkg/api/meta/restmapper.go

@@ -251,3 +251,42 @@ func (m *DefaultRESTMapper) AliasesForResource(alias string) ([]string, bool) {
 	}
 	return nil, false
 }
+
+// MultiRESTMapper is a wrapper for multiple RESTMappers.
+type MultiRESTMapper []RESTMapper
+
+// VersionAndKindForResource provides the Version and Kind  mappings for the
+// REST resources. This implementation supports multiple REST schemas and return
+// the first match.
+func (m MultiRESTMapper) VersionAndKindForResource(resource string) (defaultVersion, kind string, err error) {
+	for _, t := range m {
+		defaultVersion, kind, err = t.VersionAndKindForResource(resource)
+		if err == nil {
+			return
+		}
+	}
+	return
+}
+
+// RESTMapping provides the REST mapping for the resource based on the resource
+// kind and version. This implementation supports multiple REST schemas and
+// return the first match.
+func (m MultiRESTMapper) RESTMapping(kind string, versions ...string) (mapping *RESTMapping, err error) {
+	for _, t := range m {
+		mapping, err = t.RESTMapping(kind, versions...)
+		if err == nil {
+			return
+		}
+	}
+	return
+}
+
+// AliasesForResource finds the first alias response for the provided mappers.
+func (m MultiRESTMapper) AliasesForResource(alias string) (aliases []string, ok bool) {
+	for _, t := range m {
+		if aliases, ok = t.AliasesForResource(alias); ok {
+			return
+		}
+	}
+	return nil, false
+}