[core] Add `line-gradient` property

Porting of https://github.com/mapbox/mapbox-gl-js/pull/6303
See the link above for the description of the feature and
its limitations).

Based on patch from @lbud (Lauren Budorick).

1 files changed, 2 insertions, 0 deletions

diff --git a/platform/darwin/scripts/generate-style-code.js b/platform/darwin/scripts/generate-style-code.js
index e89a4e29a3..bc676e9cba 100755
--- a/platform/darwin/scripts/generate-style-code.js
+++ b/platform/darwin/scripts/generate-style-code.js
@@ -367,6 +367,8 @@ global.propertyReqs = function (property, propertiesByName, type) {
             return '`' + camelizeWithLeadingLowercase(req['!']) + '` is set to `nil`';
         } else {
             let name = Object.keys(req)[0];
+            if (name === 'source')
+                return 'the data source requirements are met';
             return '`' + camelizeWithLeadingLowercase(name) + '` is set to an expression that evaluates to ' + describeValue(req[name], propertiesByName[name], type);
         }
     }).join(', and ') + '. Otherwise, it is ignored.';