diff --git a/pkgs/string_scanner/CHANGELOG.md b/pkgs/string_scanner/CHANGELOG.md
index 076b73114d..4ea4c979e5 100644
--- a/pkgs/string_scanner/CHANGELOG.md
+++ b/pkgs/string_scanner/CHANGELOG.md
@@ -1,5 +1,8 @@
 ## 1.4.2-wip
 
+* Add `LineScannerMixin` to allow custom subclasses of `StringScanner` to share
+  eager line and column tracking logic.
+
 ## 1.4.1
 
 * Move to `dart-lang/tools` monorepo.
diff --git a/pkgs/string_scanner/lib/src/eager_span_scanner.dart b/pkgs/string_scanner/lib/src/eager_span_scanner.dart
index 1ccc746aca..d19d7aec2a 100644
--- a/pkgs/string_scanner/lib/src/eager_span_scanner.dart
+++ b/pkgs/string_scanner/lib/src/eager_span_scanner.dart
@@ -2,132 +2,10 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-import 'charcode.dart';
 import 'line_scanner.dart';
 import 'span_scanner.dart';
-import 'utils.dart';
-
-// TODO(nweiz): Currently this duplicates code in line_scanner.dart. Once
-// sdk#23770 is fully complete, we should move the shared code into a mixin.
-
-/// A regular expression matching newlines across platforms.
-final _newlineRegExp = RegExp(r'\r\n?|\n');
 
 /// A [SpanScanner] that tracks the line and column eagerly, like [LineScanner].
-class EagerSpanScanner extends SpanScanner {
-  @override
-  int get line => _line;
-  int _line = 0;
-
-  @override
-  int get column => _column;
-  int _column = 0;
-
-  @override
-  LineScannerState get state =>
-      _EagerSpanScannerState(this, position, line, column);
-
-  bool get _betweenCRLF => peekChar(-1) == $cr && peekChar() == $lf;
-
-  @override
-  set state(LineScannerState state) {
-    if (state is! _EagerSpanScannerState || !identical(state._scanner, this)) {
-      throw ArgumentError('The given LineScannerState was not returned by '
-          'this LineScanner.');
-    }
-
-    super.position = state.position;
-    _line = state.line;
-    _column = state.column;
-  }
-
-  @override
-  set position(int newPosition) {
-    final oldPosition = position;
-    super.position = newPosition;
-
-    if (newPosition > oldPosition) {
-      final newlines = _newlinesIn(string.substring(oldPosition, newPosition));
-      _line += newlines.length;
-      if (newlines.isEmpty) {
-        _column += newPosition - oldPosition;
-      } else {
-        _column = newPosition - newlines.last.end;
-      }
-    } else {
-      final newlines = _newlinesIn(string.substring(newPosition, oldPosition));
-      if (_betweenCRLF) newlines.removeLast();
-
-      _line -= newlines.length;
-      if (newlines.isEmpty) {
-        _column -= oldPosition - newPosition;
-      } else {
-        _column =
-            newPosition - string.lastIndexOf(_newlineRegExp, newPosition) - 1;
-      }
-    }
-  }
-
+class EagerSpanScanner extends SpanScanner with LineScannerMixin {
   EagerSpanScanner(super.string, {super.sourceUrl, super.position});
-
-  @override
-  bool scanChar(int character) {
-    if (!super.scanChar(character)) return false;
-    _adjustLineAndColumn(character);
-    return true;
-  }
-
-  @override
-  int readChar() {
-    final character = super.readChar();
-    _adjustLineAndColumn(character);
-    return character;
-  }
-
-  /// Adjusts [_line] and [_column] after having consumed [character].
-  void _adjustLineAndColumn(int character) {
-    if (character == $lf || (character == $cr && peekChar() != $lf)) {
-      _line += 1;
-      _column = 0;
-    } else {
-      _column += inSupplementaryPlane(character) ? 2 : 1;
-    }
-  }
-
-  @override
-  bool scan(Pattern pattern) {
-    if (!super.scan(pattern)) return false;
-    final firstMatch = lastMatch![0]!;
-
-    final newlines = _newlinesIn(firstMatch);
-    _line += newlines.length;
-    if (newlines.isEmpty) {
-      _column += firstMatch.length;
-    } else {
-      _column = firstMatch.length - newlines.last.end;
-    }
-
-    return true;
-  }
-
-  /// Returns a list of [Match]es describing all the newlines in [text], which
-  /// is assumed to end at [position].
-  List<Match> _newlinesIn(String text) {
-    final newlines = _newlineRegExp.allMatches(text).toList();
-    if (_betweenCRLF) newlines.removeLast();
-    return newlines;
-  }
-}
-
-/// A class representing the state of an [EagerSpanScanner].
-class _EagerSpanScannerState implements LineScannerState {
-  final EagerSpanScanner _scanner;
-  @override
-  final int position;
-  @override
-  final int line;
-  @override
-  final int column;
-
-  _EagerSpanScannerState(this._scanner, this.position, this.line, this.column);
 }
diff --git a/pkgs/string_scanner/lib/src/line_scanner.dart b/pkgs/string_scanner/lib/src/line_scanner.dart
index 111ac78f0f..23906738e1 100644
--- a/pkgs/string_scanner/lib/src/line_scanner.dart
+++ b/pkgs/string_scanner/lib/src/line_scanner.dart
@@ -6,10 +6,15 @@ import 'charcode.dart';
 import 'string_scanner.dart';
 import 'utils.dart';
 
-// Note that much of this code is duplicated in eager_span_scanner.dart.
-
-/// A subclass of [StringScanner] that tracks line and column information.
-class LineScanner extends StringScanner {
+/// A mixin that implements the line- and column-tracking functionality of
+/// [LineScanner].
+///
+/// For example:
+///
+/// class MyScanner extends StringScanner with LineScannerMixin {
+///   MyScanner(super.string);
+/// }
+mixin LineScannerMixin on StringScanner {
   /// The scanner's current (zero-based) line number.
   int get line => _line;
   int _line = 0;
@@ -119,8 +124,6 @@ class LineScanner extends StringScanner {
     }
   }
 
-  LineScanner(super.string, {super.sourceUrl, super.position});
-
   @override
   bool scanChar(int character) {
     if (!super.scanChar(character)) return false;
@@ -191,10 +194,15 @@ class LineScanner extends StringScanner {
   }
 }
 
+/// A subclass of [StringScanner] that tracks line and column information.
+class LineScanner extends StringScanner with LineScannerMixin {
+  LineScanner(super.string, {super.sourceUrl, super.position});
+}
+
 /// A class representing the state of a [LineScanner].
 class LineScannerState {
-  /// The [LineScanner] that created this.
-  final LineScanner _scanner;
+  /// The [StringScanner] that created this.
+  final StringScanner _scanner;
 
   /// The position of the scanner in this state.
   final int position;