@function can-define-backup.isDirty isDirty
@parent can-define-backup.defineMap

@description Check whether an Observe has changed since the last time it was backed up.

@signature `map.isDirty([deep])`

`isDirty` checks whether any properties have changed value or whether any properties have
been added or removed since the last time the Observe was backed up. If _deep_ is `true`,
If the Observe has never been backed up, `isDirty` returns `undefined`.
`isDirty` will include nested Observes in its checks.

@param {bool} [deep=false] whether to check nested Observes
@return {bool} Whether the Observe has changed since the last time it was [can.Map.backup.prototype.backup backed up].

@body

## Example

```js
import DefineMap from "can-define/map/map";
import defineBackup from "can-define-backup";

const recipe = new DefineMap( "Recipe", {
	title: "Pancake Mix",
	yields: "3 batches",
	ingredients: [ {
		ingredient: "flour",
		quantity: "6 cups"
	}, {
		ingredient: "baking soda",
		quantity: "1 1/2 teaspoons"
	}, {
		ingredient: "baking powder",
		quantity: "3 teaspoons"
	}, {
		ingredient: "salt",
		quantity: "1 tablespoon"
	}, {
		ingredient: "sugar",
		quantity: "2 tablespoons"
	} ]
} );
defineBackup(recipe);
recipe.isDirty();     // false
recipe.backup();

recipe.title = "Flapjack Mix";
recipe.isDirty();     // true
recipe.restore();
recipe.isDirty();   // false

recipe.ingredients.0.quantity =  "7 cups";
recipe.isDirty();     // false
recipe.isDirty(true); // true

recipe.backup();
recipe.isDirty();     // false
recipe.isDirty(true); // false
```
@highlight 25,29,31,34,35,38,39