Fix issue of previous PR

Author: mdebbar

lib/web_ui/lib/src/engine/text/paragraph.dart

@@ -383,6 +383,14 @@ class EngineParagraph implements ui.Paragraph {
       return getPositionForMultiSpanOffset(offset);
     }
 
+    // [offset] is above all the lines.
+    if (offset.dy < 0) {
+      return ui.TextPosition(
+        offset: 0,
+        affinity: ui.TextAffinity.downstream,
+      );
+    }
+
     final int lineNumber = offset.dy ~/ _measurementResult.lineHeight;
 
     // [offset] is below all the lines.

lib/web_ui/test/paragraph_test.dart

@@ -202,7 +202,7 @@ void main() async {
     );
     // Above the first line.
     expect(
-      paragraph.getPositionForOffset(Offset(0, -5)),
+      paragraph.getPositionForOffset(Offset(0, -15)),
       TextPosition(offset: 0, affinity: TextAffinity.downstream),
     );
     // At the end of the first line.