Make computed properties implicitly readOnly. Fixes #1925.

PRIMER.md

@@ -1539,6 +1539,8 @@ Polymer supports virtual properties whose values are calculated from other prope
 
 *Note, computing functions will only be called once all dependent properties are defined (`!=undefined`).  If one or more of the properties are optional, they would need default `value`'s defined in `properties` to ensure the property is computed.*
 
+Computed properties are implicitly `readOnly`, and cannot be manually set.
+
 ```html
 <dom-module id="x-custom">
   <template>
@@ -1865,7 +1867,7 @@ Example:
     <my-toolbar>My awesome app</my-toolbar>
     <button on-tap="changeTheme">Change theme</button>
   </template>
-  
+
   <script>
     Polymer({
       is: 'x-custom',

src/lib/bind/accessors.html

@@ -174,7 +174,8 @@
       // ReadOnly properties have a private setter only
       // TODO(kschaaf): Per current Bind factoring, we shouldn't
       // be interrogating the prototype here
-      if (model.getPropertyInfo && model.getPropertyInfo(property).readOnly) {
+      var info = model.getPropertyInfo && model.getPropertyInfo(property);
+      if (info && (info.readOnly || info.computed)) {
         model['_set' + this.upper(property)] = setter;
       } else {
         defun.set = setter;

src/lib/bind/effects.html

@@ -77,7 +77,7 @@
       if (args) {
         var fn = this[effect.method];
         if (fn) {
-          this[effect.property] = fn.apply(this, args);
+          this._setProperty(effect.property, fn.apply(this, args));
         } else {
           this._warn(this._logf('_computeEffect', 'compute method `' +
             effect.method + '` not defined'));

src/standard/effectBuilder.html

@@ -63,7 +63,7 @@
           if (prop.reflectToAttribute) {
             this._addPropertyEffect(p, 'reflect');
           }
-          if (prop.readOnly) {
+          if (prop.readOnly || prop.computed) {
             // Ensure accessor is created
             Polymer.Bind.ensurePropertyEffects(this, p);
           }

test/unit/bind.html

@@ -94,6 +94,14 @@
     assert.equal(el.$.boundChild.computedvalue, 45, 'Computed value not propagated to bound child');
   });
 
+  test('computed value readOnly from outside', function() {
+    el.value = 44;
+    // Should have no effect
+    el.computedvalue = 99;
+    assert.equal(el.computedvalue, 45, 'Computed value not correct');
+    assert.equal(el.$.boundChild.computedvalue, 45, 'Computed value not propagated to bound child');
+  });
+
   test('computed values to same method updates', function() {
     el.value = 44;
     el.valuetwo = 144;