[FEATURE beta] Assert key in Ember.get is not empty if '' not `in

obj`

This adds an assertion to let the user know when they are using an empty
string with an object that has no `''` property.

I'm keeping `Ember.get({'': 42}, '')` as a valid expression given it is
a tested behaviour.

Addresses [this
comment](emberjs#14572 (comment)) in emberjs#14572

packages/ember-metal/lib/property_get.js

@@ -52,6 +52,7 @@ export function get(obj, keyName) {
   assert(`Cannot call get with '${keyName}' on an undefined object.`, obj !== undefined && obj !== null);
   assert(`The key provided to get must be a string, you passed ${keyName}`, typeof keyName === 'string');
   assert(`'this' in paths is not supported`, !hasThis(keyName));
+  assert('Cannot call `Ember.get` with an empty string', keyName !== '' || '' in obj);
 
   let value = obj[keyName];
   let desc = (value !== null && typeof value === 'object' && value.isDescriptor) ? value : undefined;

packages/ember-metal/tests/accessors/get_test.js

@@ -114,6 +114,7 @@ QUnit.test('warn on attempts to use get with an unsupported property path', func
   expectAssertion(() => get(obj, undefined), /The key provided to get must be a string, you passed undefined/);
   expectAssertion(() => get(obj, false),     /The key provided to get must be a string, you passed false/);
   expectAssertion(() => get(obj, 42),        /The key provided to get must be a string, you passed 42/);
+  expectAssertion(() => get(obj, ''), /Cannot call `Ember.get` with an empty string/);
 });
 
 // ..........................................................