style(modeling): use constant instead of magic string

lib/features/modeling/behavior/LabelBehavior.js

@@ -26,6 +26,7 @@ var DEFAULT_LABEL_DIMENSIONS = {
 };
 
 var NAME_PROPERTY = 'name';
+var TEXT_PROPERTY = 'text';
 
 /**
  * A component that makes sure that external labels are added
@@ -53,7 +54,7 @@ export default function LabelBehavior(
       modeling.updateLabel(element, properties[NAME_PROPERTY]);
     }
 
-    if ('text' in properties
+    if (TEXT_PROPERTY in properties
         && is(element, 'bpmn:TextAnnotation')) {
 
       var newBounds = textRenderer.getTextAnnotationBounds(
@@ -63,7 +64,7 @@ export default function LabelBehavior(
           width: element.width,
           height: element.height
         },
-        properties.text || ''
+        properties[TEXT_PROPERTY] || ''
       );
 
       modeling.updateLabel(element, properties.text, newBounds);