Gracefully handle template literal properties in the parser

var x = {
    a: "Hello"
    `b`: 123
}

Right now, the parser encounters the template string, decides it should bail out of object literal parsing, and we more or less recover thinking we have something like the following:

var x = ({ a: "Hello" } `b`) : 123 }

So we'll report an error on what we think is:

{ a: "Hello" } `b`

• On the syntactic layer under ES5 mode, we'll say tagged templates are not supported unless in ES6.
• On the semantic layer, we'll say { a: "Hello" } is not invokable.