fix(shadow_boundary): change ShadowBoundary not to reorder styles when prepending them

Author: vsavkin

lib/core_dom/shadow_boundary.dart

@@ -16,21 +16,41 @@ abstract class ShadowBoundary {
     if (elements.isEmpty) return;
 
     final newStyles = _newStyles(elements);
-    final cloned = newStyles.map((el) => el.clone(true));
-
-    cloned.forEach((style) {
-      if (_lastStyleElement != null && !prepend) {
-        _lastStyleElement = root.insertBefore(style, _lastStyleElement.nextNode);
-      } else if (root.hasChildNodes()) {
-        _lastStyleElement = root.insertBefore(style, root.firstChild);
-      } else {
-        _lastStyleElement = root.append(style);
-      }
-    });
+    if (newStyles.isEmpty) return;
+
+    final cloned = newStyles.map((el) => el.clone(true)).toList();
+    if (_lastStyleElement == null) {
+      _insertFirstStyles(cloned);
+    } else {
+      _insertStyles(cloned, prepend);
+    }
 
     _addInsertedStyles(newStyles);
   }
 
+  _insertFirstStyles(List<dom.StyleElement> elements) {
+    elements.reversed.forEach(_insertFrontNode);
+    _lastStyleElement = elements.last;
+  }
+
+  _insertStyles(List<dom.StyleElement> elements, bool prepend) {
+    if (prepend) {
+      elements.reversed.forEach(_insertFrontNode);
+    } else {
+      final next = _lastStyleElement.nextNode;
+      root.insertAllBefore(elements, next);
+      _lastStyleElement = elements.last;
+    }
+  }
+
+  _insertFrontNode(dom.StyleElement style) {
+    if (root.hasChildNodes()) {
+      return root.insertBefore(style, root.firstChild);
+    } else {
+      return root.append(style);
+    }
+  }
+
   Iterable<dom.StyleElement> _newStyles(Iterable<dom.StyleElement> elements) {
     if (_insertedStyles == null) return elements;
     return elements.where((el) => !_insertedStyles.contains(el));

test/core_dom/shadow_boundary_spec.dart

@@ -26,7 +26,18 @@ main() {
           expect(root).toHaveText(".style1{}.style2{}.style3{}");
         });
 
-        it("should prepend style elements before other style elements", () {
+        it("should prepend style elements before other style elements (prepend, then append)", () {
+          final s1 = new dom.StyleElement()..text = ".style1{}";
+          final s2 = new dom.StyleElement()..text = ".style2{}";
+          final s3 = new dom.StyleElement()..text = ".style3{}";
+
+          boundary.insertStyleElements([s1, s2], prepend: true);
+          boundary.insertStyleElements([s3]);
+
+          expect(root).toHaveText(".style1{}.style2{}.style3{}");
+        });
+
+        it("should prepend style elements before other style elements (append, then prepend)", () {
           final s1 = new dom.StyleElement()..text = ".style1{}";
           final s2 = new dom.StyleElement()..text = ".style2{}";
           final s3 = new dom.StyleElement()..text = ".style3{}";