Ensure property values are always overridden by extendors/behaviors

lib/legacy/class.html

@@ -156,6 +156,12 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
     }
 
     function mergePropertyInfo(a, b) {
+      if ('value' in b) {
+        a.value = b.value;
+      // ensure property value is always overridden.
+      } else if ('value' in a) {
+        delete a.value;
+      }
       if (typeof b === 'function') {
         a.type = b;
         return;
@@ -163,9 +169,6 @@ subject to an additional IP rights grant found at http://polymer.github.io/PATEN
       if ('type' in b) {
         a.type = b.type;
       }
-      if ('value' in b) {
-        a.value = b.value;
-      }
       // readOnly: cannot become false and takes on `computed` value
       a.readOnly = a.readOnly || Boolean(a.computed) || b.readOnly || Boolean(b.computed);
       // computed: first in wins