fix(scopes): Support Function and Class Declarations without id

These declarations are used in combination with default exports
```
export default function () {};
```

Author: danez

src/attachScopes.js

@@ -61,7 +61,7 @@ class Scope {
 			// it's a `var` or function node, and this
 			// is a block scope, so we need to go up
 			this.parent.addDeclaration( node, isBlockDeclaration, isVar );
-		} else {
+		} else if ( node.id ) {
 			extractNames( node.id ).forEach( name => {
 				this.declarations[ name ] = true;
 			});

test/test.js

@@ -177,6 +177,41 @@ describe( 'rollup-pluginutils', function () {
 			assert.ok( scope.contains( 'bar' ) );
 		});
 
+		it( 'supports FunctionDeclarations without id', function () {
+			var ast = {
+				"type": "Program",
+				"start": 0,
+				"end": 33,
+				"body": [
+				    {
+						"type": "ExportDefaultDeclaration",
+						"start": 0,
+						"end": 32,
+						"declaration": {
+							"type": "FunctionDeclaration",
+							"start": 15,
+							"end": 32,
+							"id": null,
+							"generator": false,
+							"expression": false,
+							"async": false,
+							"params": [],
+							"body": {
+								"type": "BlockStatement",
+								"start": 26,
+								"end": 32,
+								"body": []
+					  		}
+						}
+				  	}
+				],
+				"sourceType": "module"
+			  };
+
+			var scope = attachScopes( ast, 'scope' );
+			// does not throw
+		});
+
 		// TODO more tests
 	});