Simplify helper wrapper

This improves performance slightly and gives the wrapper function a more
descriptive name to help with debugging/profiling.

lib/handlebars/helpers.js

@@ -25,3 +25,32 @@ export function moveHelperToHooks(instance, helperName, keepHelper) {
     }
   }
 }
+
+export function mergeHelpers(env, options, container) {
+  const mergedHelpers = {};
+  addHelpers(mergedHelpers, env.helpers, container);
+  addHelpers(mergedHelpers, options.helpers, container);
+  return mergedHelpers;
+}
+
+function addHelpers(mergedHelpers, helpers, container) {
+  if (!helpers) return;
+  Object.keys(helpers).forEach(helperName => {
+    const helper = helpers[helperName];
+    mergedHelpers[helperName] = passLookupPropertyOption(helper, container);
+  });
+}
+
+function passLookupPropertyOption(helper, container) {
+  if (typeof helper !== 'function') {
+    // This should not happen, but apparently it does in https://github.com/wycats/handlebars.js/issues/1639
+    // We try to make the wrapper least-invasive by not wrapping it, if the helper is not a function.
+    return helper;
+  }
+  const lookupProperty = container.lookupProperty;
+  return function invokeHelper(/* dynamic arguments */) {
+    const options = arguments[arguments.length - 1];
+    options.lookupProperty = lookupProperty;
+    return helper.apply(this, arguments);
+  };
+}

lib/handlebars/runtime.js

@@ -6,8 +6,7 @@ import {
   LAST_COMPATIBLE_COMPILER_REVISION,
   REVISION_CHANGES
 } from './base';
-import { moveHelperToHooks } from './helpers';
-import { wrapHelper } from './internal/wrapHelper';
+import { mergeHelpers, moveHelperToHooks } from './helpers';
 import {
   createProtoAccessControl,
   resultIsAllowed
@@ -240,10 +239,7 @@ export function template(templateSpec, env) {
 
   ret._setup = function(options) {
     if (!options.partial) {
-      let mergedHelpers = {};
-      addHelpers(mergedHelpers, env.helpers, container);
-      addHelpers(mergedHelpers, options.helpers, container);
-      container.helpers = mergedHelpers;
+      container.helpers = mergeHelpers(env, options, container);
 
       if (templateSpec.usePartial) {
         // Use mergeIfNeeded here to prevent compiling global partials multiple times
@@ -421,19 +417,3 @@ function executeDecorators(fn, prog, container, depths, data, blockParams) {
   }
   return prog;
 }
-
-function addHelpers(mergedHelpers, helpers, container) {
-  if (!helpers) return;
-  Object.keys(helpers).forEach(helperName => {
-    let helper = helpers[helperName];
-    mergedHelpers[helperName] = passLookupPropertyOption(helper, container);
-  });
-}
-
-function passLookupPropertyOption(helper, container) {
-  const lookupProperty = container.lookupProperty;
-  return wrapHelper(helper, options => {
-    options.lookupProperty = lookupProperty;
-    return options;
-  });
-}