Throw warning when extending an already extended map

[Macrofig]

How often can you reproduce it?

• Always
• Sometimes
• Rarely
• Unable
• I didn’t try

Description:

Extending an already extended map yield unexpected results.

Plugin example

const ViewModel = can.DefineMap.extend({
  name: {
    value: 'juan'
  }
});

const Test = ViewModel.extend({
  name() {
    return 'Juan';
  }
});

The second extended Map's should throw a warning. The eventually created instance may not behave as expected when attempting to set/get name.

Steps to reproduce:

http://jsbin.com/taxezah/2/edit?js,console

1. Create a define map using extend.
2. Then extend that map.
3. Finally, create an instance of the extended map. This becomes a little more confusing when the original map has a static value but the extended map has a getter/compute for the same property.

Expected results:

A warning should be thrown when extending an already extended map.

Actual results:

No warning is currently thrown.

Environment:

Software	Version
can-define version	1.5.1
Browser	Chrome 61.0.3163.100
Operating system	MacOS 10.13

[justinbmeyer]

I'm not sure if the warning should be on extension or at least on setting the .name property.

Note that if you are running in strict mode it does throw an error: http://jsbin.com/yimiyuz/1/edit?html,js,console