Add warning for redeclared computed properties.

lib/mixins/element-mixin.js

@@ -217,8 +217,12 @@ export const ElementMixin = dedupingMixin(base => {
     // setup where multiple triggers for setting a property)
     // While we do have `hasComputedEffect` this is set on the property's
     // dependencies rather than itself.
-    if (info.computed && !proto._hasReadOnlyEffect(name)) {
-      proto._createComputedProperty(name, info.computed, allProps);
+    if (info.computed) {
+      if (proto._hasReadOnlyEffect(name)) {
+        console.warn(`Cannot redefine computed property '${name}'.`)
+      } else {
+        proto._createComputedProperty(name, info.computed, allProps);
+      }
     }
     if (info.readOnly && !proto._hasReadOnlyEffect(name)) {
       proto._createReadOnlyProperty(name, !info.computed);

test/unit/property-effects.html

@@ -1877,7 +1877,6 @@
 suite('warn on legacy differences', () => {
 
   setup(function() {
-    setLegacyOptimizations(true);
     sinon.spy(console, 'warn');
   });
 
@@ -1887,6 +1886,7 @@
   });
 
   test('warn if non-declared property used in binding', () => {
+    setLegacyOptimizations(true);
     Polymer({
       is: 'x-warn-undeclared-binding',
       _template: html`
@@ -1900,6 +1900,25 @@
     assert.equal(console.warn.callCount, 3);
   });
 
+  test('warn when re-declaring a computed property', () => {
+    Polymer({
+      is: 'x-warn-redeclared-computed',
+      behaviors: [{
+        properties: {
+          a: { computed: 'compute(x)' }
+        }
+      }, {
+        properties: {
+          a: { computed: 'compute(y)' }
+        }
+      }]
+    });
+    const el = document.createElement('x-warn-redeclared-computed');
+    document.body.appendChild(el);
+    document.body.removeChild(el);
+    assert.equal(console.warn.callCount, 1);
+  });
+
 });
 </script>