Fix isTrailingComment to handle mixins that include loud comments.

lib/src/visitor/serialize.dart

@@ -1143,6 +1143,16 @@ class _SerializeVisitor
       // a simple forward search of the previousSpan.text as that might contain
       // other left braces.
       var searchFrom = node.span.start.offset - previousSpan.start.offset - 1;
+      if (searchFrom < 0) {
+        // This can happen when the loud comment is the first statement in a
+        // mixin that's later included. In that case, node.span.start.offset
+        // (representing the loud comment in the mixin) will be less than
+        // previousSpan.start.offset (representing the statement where the
+        // mixin is later included) ==> the loud comment cannot possibly be
+        // trailing.
+        return false;
+      }
+
       var endOffset = previousSpan.text.lastIndexOf("{", searchFrom);
       endOffset = math.max(0, endOffset);
       previousSpan = previousSpan.file.span(

test/output_test.dart

@@ -162,5 +162,16 @@ selector[href*=\"{\"] { /* please don't move me */ }
 
 @rule3;"""));
     });
+
+    test("loud comments in mixin", () {
+      expect(compileString("""
+@mixin loudComment {
+  /* ... */
+}
+
+selector {
+  @include loudComment;
+}"""), "selector {\n  /* ... */\n}");
+    });
   });
 }