add support for job outputs. (#365)

* add support for job outputs.

src/Sdk/DTObjectTemplating/ObjectTemplating/TemplateEvaluator.cs

@@ -47,7 +47,7 @@ namespace GitHub.DistributedTask.ObjectTemplating
             var evaluator = new TemplateEvaluator(context, template, removeBytes);
             try
             {
-                var availableContext = new HashSet<String>(context.ExpressionValues.Keys);
+                var availableContext = new HashSet<String>(context.ExpressionValues.Keys.Concat(context.ExpressionFunctions.Select(x => $"{x.Name}({x.MinParameters},{x.MaxParameters})")));
                 var definitionInfo = new DefinitionInfo(context.Schema, type, availableContext);
                 result = evaluator.Evaluate(definitionInfo);
 
@@ -182,12 +182,14 @@ namespace GitHub.DistributedTask.ObjectTemplating
             }
 
             var keys = new HashSet<String>(StringComparer.OrdinalIgnoreCase);
+            var hasExpressionKey = false;
 
             while (m_unraveler.AllowScalar(definition.Expand, out ScalarToken nextKeyScalar))
             {
                 // Expression
                 if (nextKeyScalar is ExpressionToken)
                 {
+                    hasExpressionKey = true;
                     var anyDefinition = new DefinitionInfo(definition, TemplateConstants.Any);
                     mapping.Add(nextKeyScalar, Evaluate(anyDefinition));
                     continue;
@@ -268,6 +270,19 @@ namespace GitHub.DistributedTask.ObjectTemplating
                 String listToDeDuplicate = String.Join(", ", nonDuplicates);
                 m_context.Error(mapping, TemplateStrings.UnableToDetermineOneOf(listToDeDuplicate));
             }
+            else if (mappingDefinitions.Count == 1 && !hasExpressionKey)
+            {
+                foreach (var property in mappingDefinitions[0].Properties)
+                {
+                    if (property.Value.Required)
+                    {
+                        if (!keys.Contains(property.Key))
+                        {
+                            m_context.Error(mapping, $"Required property is missing: {property.Key}");
+                        }
+                    }
+                }
+            }
 
             m_unraveler.ReadMappingEnd();
         }