Verify computed property dependencies

Improves the validator to fail if someone forgets to declare
dependent properties for computed state:

```
export default {
  computed: {
    bar: () => { return new Date().getTime(); }
  }
};
```

compiler/validate/js/propValidators/computed.js

@@ -21,7 +21,14 @@ export default function computed ( validator, prop ) {
 			return;
 		}
 
-		computation.value.params.forEach( param => {
+		const params = computation.value.params;
+
+		if ( params.length === 0 ) {
+			validator.error( `A computed value must depend on at least one property`, computation.value.start );
+			return;
+		}
+
+		params.forEach( param => {
 			const valid = param.type === 'Identifier' || param.type === 'AssignmentPattern' && param.left.type === 'Identifier';
 
 			if ( !valid ) {

test/validator/computed-values/errors.json

@@ -0,0 +1,8 @@
+[{
+	"message": "A computed value must depend on at least one property",
+	"pos": 49,
+	"loc": {
+		"line": 4,
+		"column": 8
+	}
+}]

test/validator/computed-values/input.html

@@ -0,0 +1,7 @@
+<script>
+	export default {
+		computed: {
+			foo: () => {}
+		}
+	};
+</script>