[AttributedText] - Fix incorrect character lookup by index (Resolves #2531) (#2532)

Author: matthew-carroll

attributed_text/lib/src/attributed_text.dart

@@ -119,13 +119,13 @@ class AttributedText {
   late final String _textWithPlaceholders;
 
   /// Returns the character or placeholder at offset zero.
-  Object get first => placeholders[0] ?? _text[0];
+  Object get first => placeholders[0] ?? _textWithPlaceholders[0];
 
   /// Returns the character or placeholder at the given [offset].
-  Object operator [](int offset) => placeholders[offset] ?? _text[offset];
+  Object operator [](int offset) => placeholders[offset] ?? _textWithPlaceholders[offset];
 
   /// Returns the character or placeholder at the end of this `AttributedText`.
-  Object get last => placeholders[length - 1] ?? _text[length - 1];
+  Object get last => placeholders[length - 1] ?? _textWithPlaceholders[length - 1];
 
   /// Returns a plain-text version of this `AttributedText`.
   ///

attributed_text/test/attributed_text_placeholders_test.dart

@@ -81,6 +81,26 @@ void main() {
       });
     });
 
+    test("reports characters and placeholders at indices", () {
+      final text1 = AttributedText("HelloWorld", null, {
+        0: const _FakePlaceholder("one"),
+        6: const _FakePlaceholder("two"),
+        12: const _FakePlaceholder("three"),
+      });
+
+      expect(text1.first, const _FakePlaceholder("one"));
+      expect(text1[1], "H");
+      expect(text1[6], const _FakePlaceholder("two"));
+      expect(text1[11], "d");
+      expect(text1.last, const _FakePlaceholder("three"));
+
+      final text2 = AttributedText("Hello World", null, {
+        0: const _FakePlaceholder("one"),
+      });
+      expect(text2[11], "d");
+      expect(text2.last, "d");
+    });
+
     test("reports plain text value", () {
       expect(
         AttributedText("", null, {