Fix view.lookup to not take dependencies

The template compiler assumes that single-segment paths like {{foo}}, compiled into view.lookup(“foo”), don’t take dependencies, returning a function if there is anything reactive going on.  We violated that in the Blaze refactor, meaning that #with might re-render its contents when its argument changes!

Tests to add:
* UI.dynamic doesn’t re-create template when enclosing data context changes.
* UI.dynamic doesn’t re-create template when “data” argument changes
* Coverage for the “..” and “foo” cases in lookup.js

Tests to fix: “simple helper”

packages/blaze/lookup.js

@@ -28,6 +28,10 @@ var wrapHelper = function (f) {
 // If a function is found, it is bound to the object it
 // was found on.  Returns a function,
 // non-function value, or null.
+//
+// NOTE: This function must not establish any reactive
+// dependencies.  If there is any reactivity in the
+// value, lookup should return a function.
 Blaze.View.prototype.lookup = function (name, _options) {
   var template = this.template;
   var lookupTemplate = _options && _options.template;
@@ -38,7 +42,7 @@ Blaze.View.prototype.lookup = function (name, _options) {
     if (!/^(\.)+$/.test(name))
       throw new Error("id starting with dot must be a series of dots");
 
-    return Blaze._parentData(name.length - 1);
+    return Blaze._parentData(name.length - 1, true /*_functionWrapped*/);
 
   } else if (template && (name in template)) {
     return wrapHelper(bindToCurrentDataIfIsFunction(template[name]));
@@ -47,29 +51,37 @@ Blaze.View.prototype.lookup = function (name, _options) {
   } else if (UI._globalHelpers[name]) {
     return wrapHelper(bindToCurrentDataIfIsFunction(UI._globalHelpers[name]));
   } else {
-    var data = Blaze.getCurrentData();
+    return function () {
-    if (data)
+      var data = Blaze.getCurrentData();
-      return wrapHelper(bindIfIsFunction(data[name], data));
+      if (lookupTemplate && ! (data && data[name]))
+        throw new Error("No such template: " + name);
+      if (! data)
+        return null;
+      var x = data[name];
+      if (typeof x !== 'function')
+        return x;
+      return x.apply(data, arguments);
+    };
   }
   return null;
 };
 
 // Implement Spacebars' {{../..}}.
 // @param height {Number} The number of '..'s
-Blaze._parentData = function (height) {
+Blaze._parentData = function (height, _functionWrapped) {
   var theWith = Blaze.getCurrentView('with');
   for (var i = 0; (i < height) && theWith; i++) {
     theWith = Blaze.getParentView(theWith, 'with');
   }
 
-  return (theWith ? theWith.dataVar.get() : null);
+  if (! theWith)
+    return null;
+  if (_functionWrapped)
+    return function () { return theWith.dataVar.get(); };
+  return theWith.dataVar.get();
 };
 
 
 Blaze.View.prototype.lookupTemplate = function (name) {
-  var result = this.lookup(name, {template:true});
+  return this.lookup(name, {template:true});
-
-  if (! result)
-    throw new Error("No such template: " + name);
-  return result;
 };