feat($compile): '=?' makes '=' binding optional

If you bind using '=' to a non-existant parent property, the compiler
will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception, which is right
because the model doesn't exist.

This enhancement allow to specify that a binding is optional so it
won't complain if the parent property is not defined. In order to mantain
backward compability, the new behaviour must be specified using '=?' instead
of '='. The local property will be undefined is these cases.

Closes #909
Closes #1435

1 files changed, 3 insertions, 1 deletions

diff --git a/docs/content/guide/directive.ngdoc b/docs/content/guide/directive.ngdoc
index 2f93ef33..925268bb 100644
--- a/docs/content/guide/directive.ngdoc
+++ b/docs/content/guide/directive.ngdoc
@@ -336,7 +336,9 @@ compiler}. The attributes are:
         Given `<widget my-attr="parentModel">` and widget definition of
         `scope: { localModel:'=myAttr' }`, then widget scope property `localModel` will reflect the
         value of `parentModel` on the parent scope. Any changes to `parentModel` will be reflected
-        in `localModel` and any changes in `localModel` will reflect in `parentModel`.
+        in `localModel` and any changes in `localModel` will reflect in `parentModel`. If the parent
+        scope property doesn't exist, it will throw a NON_ASSIGNABLE_MODEL_EXPRESSION exception. You
+        can avoid this behavior using `=?` or `=?attr` in order to flag the property as optional.
 
       * `&` or `&attr` - provides a way to execute an expression in the context of the parent scope.
         If no `attr` name is specified then the attribute name is assumed to be the same as the