Use env.VM to lookup runtime methods

Allows for overrides by 3rd parties

Fixes #679

lib/handlebars/runtime.js

@@ -27,25 +27,20 @@ export function template(templateSpec, env) {
     throw new Error("No environment passed to template");
   }
 
-  var invokePartialWrapper;
-  if (env.compile) {
-    invokePartialWrapper = function(partial, name, context, helpers, partials, data) {
-      // TODO : Check this for all inputs and the options handling (partial flag, etc). This feels
-      // like there should be a common exec path
-      var result = invokePartial.apply(this, arguments);
-      if (result != null) { return result; }
+  // Note: Using env.VM references rather than local var references throughout this section to allow
+  // for external users to override these as psuedo-supported APIs.
+  var invokePartialWrapper = function(partial, name, context, helpers, partials, data) {
+    var result = env.VM.invokePartial.apply(this, arguments);
+    if (result != null) { return result; }
 
+    if (env.compile) {
       var options = { helpers: helpers, partials: partials, data: data };
       partials[name] = env.compile(partial, { data: data !== undefined }, env);
       return partials[name](context, options);
-    };
-  } else {
-    invokePartialWrapper = function(partial, name /* , context, helpers, partials, data */) {
-      var result = invokePartial.apply(this, arguments);
-      if (result != null) { return result; }
+    } else {
       throw new Exception("The partial " + name + " could not be compiled when running in runtime-only mode");
-    };
-  }
+    }
+  };
 
   // Just add water
   var container = {
@@ -71,8 +66,8 @@ export function template(templateSpec, env) {
       }
       return ret;
     },
-    programWithDepth: programWithDepth,
-    noop: noop,
+    programWithDepth: env.VM.programWithDepth,
+    noop: env.VM.noop,
     compilerInfo: null
   };