Adds legacyUndefined setting

If `legacyUndefined` is set, multi-property observers, computed properties, and bindings will not be executed unless none of their dependencies is `undefined`. This matches Polymer 1 behavior.

lib/mixins/property-effects.js

@@ -19,12 +19,14 @@ import { camelToDashCase, dashToCamelCase } from '../utils/case-map.js';
 import { PropertyAccessors } from './property-accessors.js';
 /* for annotated effects */
 import { TemplateStamp } from './template-stamp.js';
-import { sanitizeDOMValue } from '../utils/settings.js';
+import { sanitizeDOMValue, legacyUndefined } from '../utils/settings.js';
 
 // Monotonically increasing unique ID used for de-duping effects triggered
 // from multiple properties in the same turn
 let dedupeId = 0;
 
+const NOOP = {};
+
 /**
  * Property effect types; effects are stored on the prototype using these keys
  * @enum {string}
@@ -432,6 +434,10 @@ function runComputedEffects(inst, changedProps, oldProps, hasPaths) {
  */
 function runComputedEffect(inst, property, props, oldProps, info) {
   let result = runMethodEffect(inst, property, props, oldProps, info);
+  // Abort if method returns a no-op result
+  if (result === NOOP) {
+    return;
+  }
   let computedProp = info.methodInfo;
   if (inst.__dataHasAccessor && inst.__dataHasAccessor[computedProp]) {
     inst._setPendingProperty(computedProp, result, true);
@@ -579,7 +585,10 @@ function runBindingEffect(inst, path, props, oldProps, info, hasPaths, nodeList)
   } else {
     let value = info.evaluator._evaluateBinding(inst, part, path, props, oldProps, hasPaths);
     // Propagate value to child
-    applyBindingValue(inst, node, binding, part, value);
+    // Abort if value is a no-op result
+    if (value !== NOOP) {
+      applyBindingValue(inst, node, binding, part, value);
+    }
   }
 }
 
@@ -816,7 +825,7 @@ function runMethodEffect(inst, property, props, oldProps, info) {
   let fn = context[info.methodName];
   if (fn) {
     let args = inst._marshalArgs(info.args, property, props);
-    return fn.apply(context, args);
+    return args === NOOP ? NOOP : fn.apply(context, args);
   } else if (!info.dynamicFn) {
     console.warn('method `' + info.methodName + '` not defined');
   }
@@ -2215,6 +2224,11 @@ export const PropertyEffects = dedupingMixin(superClass => {
             value = structured ? getArgValue(data, props, name) : data[name];
           }
         }
+        // When the `legacyUndefined` flag is enabled, pass a no-op value
+        // so that the observer, computed property, or compound binding is aborted.
+        if (legacyUndefined && value === undefined) {
+          return NOOP;
+        }
         values[i] = value;
       }
       return values;

lib/utils/settings.js

@@ -158,3 +158,22 @@ export let syncInitialRender = false;
 export const setSyncInitialRender = function(useSyncInitialRender) {
   syncInitialRender = useSyncInitialRender;
 };
+
+/**
+ * Setting to retain the legacy Polymer 1 behavior for multi-property
+ * observers around undefined values. Observers and computed property methods
+ * are not called until no argument is undefined.
+ */
+export let legacyUndefined = false;
+
+/**
+ * Sets `legacyUndefined` globally for all elements to enable legacy
+ * multi-property behavior for undefined values.
+ *
+ * @param {boolean} useLegacyUndefined enable or disable legacy
+ * multi-property behavior for undefined.
+ * @return {void}
+ */
+export const setLegacyUndefined = function(useLegacyUndefined) {
+  legacyUndefined = useLegacyUndefined;
+};