Raise error on duplicate definition of __proto__ (#183)

Spec: http://www.ecma-international.org/ecma-262/6.0/#sec-__proto__-property-names-in-object-initializers

This brings back to life a bit of dead code in Parser.prototype.checkPropClash, and enables several tests that were previously disabled.

src/parser/expression.js

@@ -50,7 +50,7 @@ pp.checkPropClash = function (prop, propHash) {
       return;
   }
 
-  if (name === "__proto__" && prop.kind === "init") {
+  if (name === "__proto__" && !prop.kind) {
     if (propHash.proto) this.raise(key.start, "Redefinition of __proto__ property");
     propHash.proto = true;
   }