[web] Fix exception when getting boxes for rich text range (#17933)

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

@@ -359,13 +359,23 @@ class EngineParagraph implements ui.Paragraph {
   }) {
     assert(boxHeightStyle != null);
     assert(boxWidthStyle != null);
-    if (_plainText == null || start == end) {
+    // Zero-length ranges and invalid ranges return an empty list.
+    if (start == end || start < 0 || end < 0) {
       return <ui.TextBox>[];
     }
 
+    // For rich text, we can't measure the boxes. So for now, we'll just return
+    // a placeholder box to stop exceptions from being thrown in the framework.
+    // https://github.com/flutter/flutter/issues/55587
+    if (_plainText == null) {
+      return <ui.TextBox>[
+        ui.TextBox.fromLTRBD(0, 0, 0, _lineHeight, _textDirection)
+      ];
+    }
+
     final int length = _plainText.length;
     // Ranges that are out of bounds should return an empty list.
-    if (start < 0 || end < 0 || start > length || end > length) {
+    if (start > length || end > length) {
       return <ui.TextBox>[];
     }
 

lib/web_ui/test/paragraph_test.dart

@@ -415,6 +415,25 @@ void main() async {
     );
   });
 
+  testEachMeasurement('getBoxesForRange returns a box for rich text', () {
+    final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(
+      fontFamily: 'Ahem',
+      fontStyle: FontStyle.normal,
+      fontWeight: FontWeight.normal,
+      fontSize: 10,
+      textDirection: TextDirection.ltr,
+    ));
+    builder.addText('abcd');
+    builder.pushStyle(TextStyle(fontWeight: FontWeight.bold));
+    builder.addText('xyz');
+    final Paragraph paragraph = builder.build();
+    paragraph.layout(const ParagraphConstraints(width: 1000));
+    expect(
+      paragraph.getBoxesForRange(1, 2).single,
+      const TextBox.fromLTRBD(0, 0, 0, 10, TextDirection.ltr),
+    );
+  });
+
   testEachMeasurement(
       'getBoxesForRange return empty list for zero-length range', () {
     final ParagraphBuilder builder = ParagraphBuilder(ParagraphStyle(