[BUGFIX LTS] Warn about invalid properties

There was a bug in the brace expansion implementation before 2.13 that
admitted expressions like `a.{b,c` and `a.b,c` in property expansion
without failing.

On February 2017, emberjs#13231 was merged improving the implementation of
`expandProperties` and fixing the previous issue.

Sadly, people upgrading from 2.12 are having problems with this. We have
two options here:

- Backporting emberjs#13231 fixes it and can be done, as of today, without any
  problem just by cherry-picking its two commits. Sadly, this would
  break people apps in a patch version, and this bug does not seem like
  a critical bugfix.

- Adding a warning if a user is using invalid properties. I went with
  this option since it looks like the less intrusive one while letting
  the users know that their apps will break in the next update.

Author: Serabe

packages/ember-metal/lib/expand_properties.js

@@ -1,4 +1,4 @@
-import { assert } from './debug';
+import { assert, warn } from './debug';
 
 /**
 @module ember
@@ -63,6 +63,30 @@ export default function expandProperties(pattern, callback) {
       return true;
     })(pattern));
 
+  warn(
+    `You used the key "${pattern}" which is invalid and is not working starting on Ember 2.13.`,
+    ((str) => {
+      let inBrace = 0;
+      let char;
+      for (let i = 0; i < str.length; i++) {
+        char = str.charAt(i);
+
+        if (char === '{') {
+          inBrace++;
+        } else if (char === '}') {
+          inBrace--;
+        } else if (char === ',' && inBrace === 0) {
+          return false;
+        }
+
+        if (inBrace > 1 || inBrace < 0) {
+          return false;
+        }
+      }
+
+      return inBrace === 0;
+    })(pattern), { id: 'ember-metal.invalid-expand-properties' });
+
   let parts = pattern.split(SPLIT_REGEX);
   let properties = [parts];
 

packages/ember-metal/tests/expand_properties_test.js

@@ -97,6 +97,19 @@ QUnit.test('A pattern must be a string', function() {
   }, /A computed property key must be a string/);
 });
 
+QUnit.test('unbalanced braces are warned', function() {
+  let invalidBraceProperties = [
+    'model.{baz,bar',
+    'model.bar,bar'
+  ];
+
+  invalidBraceProperties.forEach((invalidProperties) =>
+    expectWarning(() => {
+      expandProperties('model.{baz,bar', addProperty)
+    }, / which is invalid and is not working starting on Ember 2\.13\./)
+  );
+});
+
 QUnit.test('A pattern must not contain a space', function() {
   expect(1);