Add test case for importer callback order

Closes #998

test/api.js

@@ -258,6 +258,26 @@ describe('api', function() {
       });
     });
 
+    it('should should resolve imports depth first', function (done) {
+      var actualImportOrder = [];
+      var expectedImportOrder = [
+        'a', '_common', 'vars', 'struct', 'a1', 'common', 'vars', 'struct', 'b', 'b1'
+      ];
+      var expected = read(fixture('depth-first/expected.css'));
+
+      sass.render({
+        file: fixture('depth-first/index.scss'),
+        importer: function (url, prev, done) {
+          actualImportOrder.push(url);
+          done();
+        }
+      }, function(error, result) {
+        assert.equal(result.css.toString().trim(), expected);
+        assert.deepEqual(actualImportOrder, expectedImportOrder);
+        done();
+      });
+    });
+
     it('should override imports with "file" as input and fires callback with file and contents', function(done) {
       sass.render({
         file: fixture('include-files/index.scss'),

test/fixtures/depth-first/_common.scss

+@import "vars";
+@import "struct";
+
+.myvars {
+    content: quote($import_counter);
+}

test/fixtures/depth-first/_struct.scss

+.common-struct {
+    content: "common-struct";
+}

test/fixtures/depth-first/_vars.scss

+$import_counter: $import_counter + 1;
+
+.common-vars {
+    content: "common-vars";
+}

test/fixtures/depth-first/a.scss

+@import "_common";
+@import "a1";
+
+.a2 {
+    content: "a2";
+}
+

test/fixtures/depth-first/a1.scss

+.a1 {
+    content: "a1";
+}

test/fixtures/depth-first/b.scss

+@import "b1";
+
+.b2 {
+    content: "b2";
+}

test/fixtures/depth-first/b1.scss

+.b1 {
+    content: "b1";
+}

test/fixtures/depth-first/expected.css

+.common-vars {
+  content: "common-vars"; }
+
+.common-struct {
+  content: "common-struct"; }
+
+.myvars {
+  content: "1"; }
+
+.a1 {
+  content: "a1"; }
+
+.a2 {
+  content: "a2"; }
+
+.common-vars {
+  content: "common-vars"; }
+
+.common-struct {
+  content: "common-struct"; }
+
+.myvars {
+  content: "2"; }
+
+.b1 {
+  content: "b1"; }
+
+.b2 {
+  content: "b2"; }
+
+#the-last {
+  content: "LAST"; }
\ No newline at end of file

test/fixtures/depth-first/index.scss

+$import_counter: 0;
+@import "a";
+@import "common";
+@import "b";
+
+#the-last {
+    content: "LAST";
+}