diff --git a/.github/ISSUE_TEMPLATE/string_scanner.md b/.github/ISSUE_TEMPLATE/string_scanner.md
new file mode 100644
index 0000000..ad89f1b
--- /dev/null
+++ b/.github/ISSUE_TEMPLATE/string_scanner.md
@@ -0,0 +1,5 @@
+---
+name: "package:string_scanner"
+about: "Create a bug or file a feature request against package:string_scanner."
+labels: "package:string_scanner"
+---
\ No newline at end of file
diff --git a/pkgs/string_scanner/.github/dependabot.yml b/pkgs/string_scanner/.github/dependabot.yml
new file mode 100644
index 0000000..a19a66a
--- /dev/null
+++ b/pkgs/string_scanner/.github/dependabot.yml
@@ -0,0 +1,16 @@
+# Set update schedule for GitHub Actions
+# See https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/keeping-your-actions-up-to-date-with-dependabot
+
+version: 2
+updates:
+
+- package-ecosystem: github-actions
+  directory: /
+  schedule:
+    interval: monthly
+  labels:
+    - autosubmit
+  groups:
+    github-actions:
+      patterns:
+        - "*"
diff --git a/pkgs/string_scanner/.github/workflows/publish.yaml b/pkgs/string_scanner/.github/workflows/publish.yaml
new file mode 100644
index 0000000..27157a0
--- /dev/null
+++ b/pkgs/string_scanner/.github/workflows/publish.yaml
@@ -0,0 +1,17 @@
+# A CI configuration to auto-publish pub packages.
+
+name: Publish
+
+on:
+  pull_request:
+    branches: [ master ]
+  push:
+    tags: [ 'v[0-9]+.[0-9]+.[0-9]+' ]
+
+jobs:
+  publish:
+    if: ${{ github.repository_owner == 'dart-lang' }}
+    uses: dart-lang/ecosystem/.github/workflows/publish.yaml@main
+    permissions:
+      id-token: write # Required for authentication using OIDC
+      pull-requests: write # Required for writing the pull request note
diff --git a/pkgs/string_scanner/.github/workflows/test-package.yml b/pkgs/string_scanner/.github/workflows/test-package.yml
new file mode 100644
index 0000000..c60f710
--- /dev/null
+++ b/pkgs/string_scanner/.github/workflows/test-package.yml
@@ -0,0 +1,64 @@
+name: Dart CI
+
+on:
+  # Run on PRs and pushes to the default branch.
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  schedule:
+    - cron: "0 0 * * 0"
+
+env:
+  PUB_ENVIRONMENT: bot.github
+
+jobs:
+  # Check code formatting and static analysis on a single OS (linux)
+  # against Dart dev.
+  analyze:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        sdk: [dev]
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+      - uses: dart-lang/setup-dart@e630b99d28a3b71860378cafdc2a067c71107f94
+        with:
+          sdk: ${{ matrix.sdk }}
+      - id: install
+        name: Install dependencies
+        run: dart pub get
+      - name: Check formatting
+        run: dart format --output=none --set-exit-if-changed .
+        if: always() && steps.install.outcome == 'success'
+      - name: Analyze code
+        run: dart analyze --fatal-infos
+        if: always() && steps.install.outcome == 'success'
+
+  # Run tests on a matrix consisting of two dimensions:
+  # 1. OS: ubuntu-latest, (macos-latest, windows-latest)
+  # 2. release channel: dev
+  test:
+    needs: analyze
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        # Add macos-latest and/or windows-latest if relevant for this package.
+        os: [ubuntu-latest]
+        sdk: [3.1, dev]
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683
+      - uses: dart-lang/setup-dart@e630b99d28a3b71860378cafdc2a067c71107f94
+        with:
+          sdk: ${{ matrix.sdk }}
+      - id: install
+        name: Install dependencies
+        run: dart pub get
+      - name: Run VM tests
+        run: dart test --platform vm
+        if: always() && steps.install.outcome == 'success'
+      - name: Run Chrome tests
+        run: dart test --platform chrome
+        if: always() && steps.install.outcome == 'success'
diff --git a/pkgs/string_scanner/.gitignore b/pkgs/string_scanner/.gitignore
new file mode 100644
index 0000000..fb97bde
--- /dev/null
+++ b/pkgs/string_scanner/.gitignore
@@ -0,0 +1,5 @@
+# Don’t commit the following directories created by pub.
+.dart_tool/
+.pub/
+.packages
+pubspec.lock
diff --git a/pkgs/string_scanner/CHANGELOG.md b/pkgs/string_scanner/CHANGELOG.md
new file mode 100644
index 0000000..082e9f2
--- /dev/null
+++ b/pkgs/string_scanner/CHANGELOG.md
@@ -0,0 +1,175 @@
+## 1.4.1
+
+* Move to `dart-lang/tools` monorepo.
+
+## 1.4.0
+
+* Fix `LineScanner`'s handling of `\r\n`'s to preventing errors scanning
+  zero-length matches when between CR and LF. CR is treated as a new line only
+  if not immediately followed by a LF.
+* Fix `LineScanner`'s updating of `column` when setting `position` if the
+  current position is not `0`.
+
+## 1.3.0
+
+* Require Dart 3.1.0
+
+* Add a `SpanScanner.spanFromPosition()` method which takes raw code units
+  rather than `SpanScanner.spanFrom()`'s `LineScannerState`s.
+
+## 1.2.0
+
+* Require Dart 2.18.0
+
+* Add better support for reading code points in the Unicode supplementary plane:
+
+  * Added `StringScanner.readCodePoint()`, which consumes an entire Unicode code
+    point even if it's represented by two UTF-16 code units.
+
+  * Added `StringScanner.peekCodePoint()`, which returns an entire Unicode code
+    point even if it's represented by two UTF-16 code units.
+
+  * `StringScanner.scanChar()` and `StringScanner.expectChar()` will now
+    properly consume two UTF-16 code units if they're passed Unicode code points
+    in the supplementary plane.
+
+## 1.1.1
+
+* Populate the pubspec `repository` field.
+* Switch to `package:lints`.
+* Remove a dependency on `package:charcode`.
+
+## 1.1.0
+
+* Stable release for null safety.
+
+## 1.1.0-nullsafety.3
+
+* Update SDK constraints to `>=2.12.0-0 <3.0.0` based on beta release
+  guidelines.
+
+## 1.1.0-nullsafety.2
+
+* Allow prerelease versions of the 2.12 sdk.
+
+## 1.1.0-nullsafety.1
+
+- Allow 2.10 stable and 2.11.0 dev SDK versions.
+
+## 1.1.0-nullsafety
+
+- Migrate to null safety.
+
+## 1.0.5
+
+- Added an example.
+
+- Update Dart SDK constraint to `>=2.0.0 <3.0.0`.
+
+## 1.0.4
+
+* Add @alwaysThrows annotation to error method.
+
+## 1.0.3
+
+* Set max SDK version to `<3.0.0`, and adjust other dependencies.
+
+## 1.0.2
+
+* `SpanScanner` no longer crashes when creating a span that contains a UTF-16
+  surrogate pair.
+
+## 1.0.1
+
+* Fix the error text emitted by `StringScanner.expectChar()`.
+
+## 1.0.0
+
+* **Breaking change**: `StringScanner.error()`'s `length` argument now defaults
+  to `0` rather than `1` when no match data is available.
+
+* **Breaking change**: `StringScanner.lastMatch` and related methods are now
+  reset when the scanner's position changes without producing a new match.
+
+**Note**: While the changes in `1.0.0` are user-visible, they're unlikely to
+actually break any code in practice. Unless you know that your package is
+incompatible with 0.1.x, consider using 0.1.5 as your lower bound rather
+than 1.0.0. For example, `string_scanner: ">=0.1.5 <2.0.0"`.
+
+## 0.1.5
+
+* Add `new SpanScanner.within()`, which scans within a existing `FileSpan`.
+
+* Add `StringScanner.scanChar()` and `StringScanner.expectChar()`.
+
+## 0.1.4+1
+
+* Remove the dependency on `path`, since we don't actually import it.
+
+## 0.1.4
+
+* Add `new SpanScanner.eager()` for creating a `SpanScanner` that eagerly
+  computes its current line and column numbers.
+
+## 0.1.3+2
+
+* Fix `LineScanner`'s handling of carriage returns to match that of
+  `SpanScanner`.
+
+## 0.1.3+1
+
+* Fixed the homepage URL.
+
+## 0.1.3
+
+* Add an optional `endState` argument to `SpanScanner.spanFrom`.
+
+## 0.1.2
+
+* Add `StringScanner.substring`, which returns a substring of the source string.
+
+## 0.1.1
+
+* Declare `SpanScanner`'s exposed `SourceSpan`s and `SourceLocation`s to be
+  `FileSpan`s and `FileLocation`s. They always were underneath, but callers may
+  now rely on it.
+
+* Add `SpanScanner.location`, which returns the scanner's current
+  `SourceLocation`.
+
+## 0.1.0
+
+* Switch from `source_maps`' `Span` class to `source_span`'s `SourceSpan` class.
+
+* `new StringScanner()`'s `sourceUrl` parameter is now named to make it clear
+  that it can be safely `null`.
+
+* `new StringScannerException()` takes different arguments in a different order
+  to match `SpanFormatException`.
+
+* `StringScannerException.string` has been renamed to
+  `StringScannerException.source` to match the `FormatException` interface.
+
+## 0.0.3
+
+* Make `StringScannerException` inherit from source_map's `SpanFormatException`.
+
+## 0.0.2
+
+* `new StringScanner()` now takes an optional `sourceUrl` argument that provides
+  the URL of the source file. This is used for error reporting.
+
+* Add `StringScanner.readChar()` and `StringScanner.peekChar()` methods for
+  doing character-by-character scanning.
+
+* Scanners now throw `StringScannerException`s which provide more detailed
+  access to information about the errors that were thrown and can provide
+  terminal-colored messages.
+
+* Add a `LineScanner` subclass of `StringScanner` that automatically tracks line
+  and column information of the text being scanned.
+
+* Add a `SpanScanner` subclass of `LineScanner` that exposes matched ranges as
+  [source map][] `Span` objects.
+
+[source_map]: https://pub.dev/packages/source_maps
diff --git a/pkgs/string_scanner/LICENSE b/pkgs/string_scanner/LICENSE
new file mode 100644
index 0000000..000cd7b
--- /dev/null
+++ b/pkgs/string_scanner/LICENSE
@@ -0,0 +1,27 @@
+Copyright 2014, the Dart project authors. 
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+    * Redistributions of source code must retain the above copyright
+      notice, this list of conditions and the following disclaimer.
+    * Redistributions in binary form must reproduce the above
+      copyright notice, this list of conditions and the following
+      disclaimer in the documentation and/or other materials provided
+      with the distribution.
+    * Neither the name of Google LLC nor the names of its
+      contributors may be used to endorse or promote products derived
+      from this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
diff --git a/pkgs/string_scanner/README.md b/pkgs/string_scanner/README.md
new file mode 100644
index 0000000..e06e325
--- /dev/null
+++ b/pkgs/string_scanner/README.md
@@ -0,0 +1,41 @@
+[![Dart CI](https://github.com/dart-lang/string_scanner/actions/workflows/test-package.yml/badge.svg)](https://github.com/dart-lang/string_scanner/actions/workflows/test-package.yml)
+[![pub package](https://img.shields.io/pub/v/string_scanner.svg)](https://pub.dev/packages/string_scanner)
+[![package publisher](https://img.shields.io/pub/publisher/string_scanner.svg)](https://pub.dev/packages/string_scanner/publisher)
+
+This package exposes a `StringScanner` type that makes it easy to parse a string
+using a series of `Pattern`s. For example:
+
+```dart
+import 'dart:math' as math;
+
+import 'package:string_scanner/string_scanner.dart';
+
+num parseNumber(String source) {
+  // Scan a number ("1", "1.5", "-3").
+  final scanner = StringScanner(source);
+
+  // [Scanner.scan] tries to consume a [Pattern] and returns whether or not it
+  // succeeded. It will move the scan pointer past the end of the pattern.
+  final negative = scanner.scan('-');
+
+  // [Scanner.expect] consumes a [Pattern] and throws a [FormatError] if it
+  // fails. Like [Scanner.scan], it will move the scan pointer forward.
+  scanner.expect(RegExp(r'\d+'));
+
+  // [Scanner.lastMatch] holds the [MatchData] for the most recent call to
+  // [Scanner.scan], [Scanner.expect], or [Scanner.matches].
+  var number = num.parse(scanner.lastMatch![0]!);
+
+  if (scanner.scan('.')) {
+    scanner.expect(RegExp(r'\d+'));
+    final decimal = scanner.lastMatch![0]!;
+    number += int.parse(decimal) / math.pow(10, decimal.length);
+  }
+
+  // [Scanner.expectDone] will throw a [FormatError] if there's any input that
+  // hasn't yet been consumed.
+  scanner.expectDone();
+
+  return (negative ? -1 : 1) * number;
+}
+```
diff --git a/pkgs/string_scanner/analysis_options.yaml b/pkgs/string_scanner/analysis_options.yaml
new file mode 100644
index 0000000..59f763a
--- /dev/null
+++ b/pkgs/string_scanner/analysis_options.yaml
@@ -0,0 +1,32 @@
+# https://dart.dev/guides/language/analysis-options
+include: package:dart_flutter_team_lints/analysis_options.yaml
+
+analyzer:
+  language:
+    strict-casts: true
+    strict-inference: true
+    strict-raw-types: true
+
+linter:
+  rules:
+    - avoid_bool_literals_in_conditional_expressions
+    - avoid_classes_with_only_static_members
+    - avoid_private_typedef_functions
+    - avoid_redundant_argument_values
+    - avoid_returning_this
+    - avoid_unused_constructor_parameters
+    - avoid_void_async
+    - cancel_subscriptions
+    - join_return_with_assignment
+    - literal_only_boolean_expressions
+    - missing_whitespace_between_adjacent_strings
+    - no_adjacent_strings_in_list
+    - no_runtimeType_toString
+    - prefer_const_declarations
+    - prefer_expression_function_bodies
+    - prefer_final_locals
+    - unnecessary_await_in_return
+    - unnecessary_raw_strings
+    - use_if_null_to_convert_nulls_to_bools
+    - use_raw_strings
+    - use_string_buffers
diff --git a/pkgs/string_scanner/example/example.dart b/pkgs/string_scanner/example/example.dart
new file mode 100644
index 0000000..ec9dd76
--- /dev/null
+++ b/pkgs/string_scanner/example/example.dart
@@ -0,0 +1,40 @@
+// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
+// 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 'dart:math' as math;
+
+import 'package:string_scanner/string_scanner.dart';
+
+void main(List<String> args) {
+  print(parseNumber(args.single));
+}
+
+num parseNumber(String source) {
+  // Scan a number ("1", "1.5", "-3").
+  final scanner = StringScanner(source);
+
+  // [Scanner.scan] tries to consume a [Pattern] and returns whether or not it
+  // succeeded. It will move the scan pointer past the end of the pattern.
+  final negative = scanner.scan('-');
+
+  // [Scanner.expect] consumes a [Pattern] and throws a [FormatError] if it
+  // fails. Like [Scanner.scan], it will move the scan pointer forward.
+  scanner.expect(RegExp(r'\d+'));
+
+  // [Scanner.lastMatch] holds the [MatchData] for the most recent call to
+  // [Scanner.scan], [Scanner.expect], or [Scanner.matches].
+  var number = num.parse(scanner.lastMatch![0]!);
+
+  if (scanner.scan('.')) {
+    scanner.expect(RegExp(r'\d+'));
+    final decimal = scanner.lastMatch![0]!;
+    number += int.parse(decimal) / math.pow(10, decimal.length);
+  }
+
+  // [Scanner.expectDone] will throw a [FormatError] if there's any input that
+  // hasn't yet been consumed.
+  scanner.expectDone();
+
+  return (negative ? -1 : 1) * number;
+}
diff --git a/pkgs/string_scanner/lib/src/charcode.dart b/pkgs/string_scanner/lib/src/charcode.dart
new file mode 100644
index 0000000..d157749
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/charcode.dart
@@ -0,0 +1,24 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+/// Character '\'.
+const int $backslash = 0x5C;
+
+/// "Carriage return" control character.
+const int $cr = 0x0D;
+
+/// Character '"'.
+const int $doubleQuote = 0x22;
+
+/// Character 'f'.
+const int $f = 0x66;
+
+/// "Line feed" control character.
+const int $lf = 0x0A;
+
+/// Space character.
+const int $space = 0x20;
+
+/// Character 'x'.
+const int $x = 0x78;
diff --git a/pkgs/string_scanner/lib/src/eager_span_scanner.dart b/pkgs/string_scanner/lib/src/eager_span_scanner.dart
new file mode 100644
index 0000000..1ccc746
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/eager_span_scanner.dart
@@ -0,0 +1,133 @@
+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// 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;
+      }
+    }
+  }
+
+  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/exception.dart b/pkgs/string_scanner/lib/src/exception.dart
new file mode 100644
index 0000000..57af541
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/exception.dart
@@ -0,0 +1,21 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:source_span/source_span.dart';
+
+import 'string_scanner.dart';
+
+/// An exception thrown by a [StringScanner] that failed to parse a string.
+class StringScannerException extends SourceSpanFormatException {
+  @override
+  String get source => super.source as String;
+
+  /// The URL of the source file being parsed.
+  ///
+  /// This may be `null`, indicating that the source URL is unknown.
+  Uri? get sourceUrl => span?.sourceUrl;
+
+  StringScannerException(
+      super.message, SourceSpan super.span, String super.source);
+}
diff --git a/pkgs/string_scanner/lib/src/line_scanner.dart b/pkgs/string_scanner/lib/src/line_scanner.dart
new file mode 100644
index 0000000..b18d610
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/line_scanner.dart
@@ -0,0 +1,183 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'string_scanner.dart';
+import 'utils.dart';
+
+// Note that much of this code is duplicated in eager_span_scanner.dart.
+
+/// A regular expression matching newlines. A newline is either a `\n`, a `\r\n`
+/// or a `\r` that is not immediately followed by a `\n`.
+final _newlineRegExp = RegExp(r'\n|\r\n|\r(?!\n)');
+
+/// A subclass of [StringScanner] that tracks line and column information.
+class LineScanner extends StringScanner {
+  /// The scanner's current (zero-based) line number.
+  int get line => _line;
+  int _line = 0;
+
+  /// The scanner's current (zero-based) column number.
+  int get column => _column;
+  int _column = 0;
+
+  /// The scanner's state, including line and column information.
+  ///
+  /// This can be used to efficiently save and restore the state of the scanner
+  /// when backtracking. A given [LineScannerState] is only valid for the
+  /// [LineScanner] that created it.
+  ///
+  /// This does not include the scanner's match information.
+  LineScannerState get state =>
+      LineScannerState._(this, position, line, column);
+
+  /// Whether the current position is between a CR character and an LF
+  /// charactet.
+  bool get _betweenCRLF => peekChar(-1) == $cr && peekChar() == $lf;
+
+  set state(LineScannerState state) {
+    if (!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) {
+    if (newPosition == position) {
+      return;
+    }
+
+    final oldPosition = position;
+    super.position = newPosition;
+
+    if (newPosition == 0) {
+      _line = 0;
+      _column = 0;
+    } else if (newPosition > oldPosition) {
+      final newlines = _newlinesIn(string.substring(oldPosition, newPosition),
+          endPosition: newPosition);
+      _line += newlines.length;
+      if (newlines.isEmpty) {
+        _column += newPosition - oldPosition;
+      } else {
+        // The regex got a substring, so we need to account for where it started
+        // in the string.
+        final offsetOfLastNewline = oldPosition + newlines.last.end;
+        _column = newPosition - offsetOfLastNewline;
+      }
+    } else if (newPosition < oldPosition) {
+      final newlines = _newlinesIn(string.substring(newPosition, oldPosition),
+          endPosition: oldPosition);
+
+      _line -= newlines.length;
+      if (newlines.isEmpty) {
+        _column -= oldPosition - newPosition;
+      } else {
+        // To compute the new column, we need to locate the last newline before
+        // the new position. When searching, we must exclude the CR if we're
+        // between a CRLF because it's not considered a newline.
+        final crOffset = _betweenCRLF ? -1 : 0;
+        // Additionally, if we use newPosition as the end of the search and the
+        // character at that position itself (the next character) is a newline
+        // we should not use it, so also offset to account for that.
+        const currentCharOffset = -1;
+        final lastNewline = string.lastIndexOf(
+            _newlineRegExp, newPosition + currentCharOffset + crOffset);
+
+        // Now we need to know the offset after the newline. This is the index
+        // above plus the length of the newline (eg. if we found `\r\n`) we need
+        // to add two. However if no newline was found, that index is 0.
+        final offsetAfterLastNewline = lastNewline == -1
+            ? 0
+            : string[lastNewline] == '\r' && string[lastNewline + 1] == '\n'
+                ? lastNewline + 2
+                : lastNewline + 1;
+
+        _column = newPosition - offsetAfterLastNewline;
+      }
+    }
+  }
+
+  LineScanner(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 newlines = _newlinesIn(lastMatch![0]!, endPosition: position);
+    _line += newlines.length;
+    if (newlines.isEmpty) {
+      _column += lastMatch![0]!.length;
+    } else {
+      _column = lastMatch![0]!.length - newlines.last.end;
+    }
+
+    return true;
+  }
+
+  /// Returns a list of [Match]es describing all the newlines in [text], which
+  /// ends at [endPosition].
+  ///
+  /// If [text] ends with `\r`, it will only be treated as a newline if the next
+  /// character at [position] is not a `\n`.
+  List<Match> _newlinesIn(String text, {required int endPosition}) {
+    final newlines = _newlineRegExp.allMatches(text).toList();
+    // If the last character is a `\r` it will have been treated as a newline,
+    // but this is only valid if the next character is not a `\n`.
+    if (endPosition < string.length &&
+        text.endsWith('\r') &&
+        string[endPosition] == '\n') {
+      // newlines should never be empty here, because if `text` ends with `\r`
+      // it would have matched `\r(?!\n)` in the newline regex.
+      newlines.removeLast();
+    }
+    return newlines;
+  }
+}
+
+/// A class representing the state of a [LineScanner].
+class LineScannerState {
+  /// The [LineScanner] that created this.
+  final LineScanner _scanner;
+
+  /// The position of the scanner in this state.
+  final int position;
+
+  /// The zero-based line number of the scanner in this state.
+  final int line;
+
+  /// The zero-based column number of the scanner in this state.
+  final int column;
+
+  LineScannerState._(this._scanner, this.position, this.line, this.column);
+}
diff --git a/pkgs/string_scanner/lib/src/relative_span_scanner.dart b/pkgs/string_scanner/lib/src/relative_span_scanner.dart
new file mode 100644
index 0000000..cd9af0e
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/relative_span_scanner.dart
@@ -0,0 +1,132 @@
+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:source_span/source_span.dart';
+
+import 'exception.dart';
+import 'line_scanner.dart';
+import 'span_scanner.dart';
+import 'string_scanner.dart';
+import 'utils.dart';
+
+/// A [SpanScanner] that scans within an existing [FileSpan].
+///
+/// This re-implements chunks of [SpanScanner] rather than using a dummy span or
+/// inheritance because scanning is often a performance-critical operation, so
+/// it's important to avoid adding extra overhead when relative scanning isn't
+/// needed.
+class RelativeSpanScanner extends StringScanner implements SpanScanner {
+  /// The source of the scanner.
+  ///
+  /// This caches line break information and is used to generate [SourceSpan]s.
+  final SourceFile _sourceFile;
+
+  /// The start location of the span within which this scanner is scanning.
+  ///
+  /// This is used to convert between span-relative and file-relative fields.
+  final FileLocation _startLocation;
+
+  @override
+  int get line =>
+      _sourceFile.getLine(_startLocation.offset + position) -
+      _startLocation.line;
+
+  @override
+  int get column {
+    final line = _sourceFile.getLine(_startLocation.offset + position);
+    final column =
+        _sourceFile.getColumn(_startLocation.offset + position, line: line);
+    return line == _startLocation.line
+        ? column - _startLocation.column
+        : column;
+  }
+
+  @override
+  LineScannerState get state => _SpanScannerState(this, position);
+
+  @override
+  set state(LineScannerState state) {
+    if (state is! _SpanScannerState || !identical(state._scanner, this)) {
+      throw ArgumentError('The given LineScannerState was not returned by '
+          'this LineScanner.');
+    }
+
+    position = state.position;
+  }
+
+  @override
+  FileSpan? get lastSpan => _lastSpan;
+  FileSpan? _lastSpan;
+
+  @override
+  FileLocation get location =>
+      _sourceFile.location(_startLocation.offset + position);
+
+  @override
+  FileSpan get emptySpan => location.pointSpan();
+
+  RelativeSpanScanner(FileSpan span)
+      : _sourceFile = span.file,
+        _startLocation = span.start,
+        super(span.text, sourceUrl: span.sourceUrl);
+
+  @override
+  FileSpan spanFrom(LineScannerState startState, [LineScannerState? endState]) {
+    final endPosition = endState == null ? position : endState.position;
+    return _sourceFile.span(_startLocation.offset + startState.position,
+        _startLocation.offset + endPosition);
+  }
+
+  @override
+  FileSpan spanFromPosition(int startPosition, [int? endPosition]) {
+    RangeError.checkValidRange(
+        startPosition,
+        endPosition,
+        _sourceFile.length - _startLocation.offset,
+        'startPosition',
+        'endPosition');
+    return _sourceFile.span(_startLocation.offset + startPosition,
+        _startLocation.offset + (endPosition ?? position));
+  }
+
+  @override
+  bool matches(Pattern pattern) {
+    if (!super.matches(pattern)) {
+      _lastSpan = null;
+      return false;
+    }
+
+    _lastSpan = _sourceFile.span(_startLocation.offset + position,
+        _startLocation.offset + lastMatch!.end);
+    return true;
+  }
+
+  @override
+  Never error(String message, {Match? match, int? position, int? length}) {
+    validateErrorArgs(string, match, position, length);
+
+    if (match == null && position == null && length == null) match = lastMatch;
+    position ??= match == null ? this.position : match.start;
+    length ??= match == null ? 1 : match.end - match.start;
+
+    final span = _sourceFile.span(_startLocation.offset + position,
+        _startLocation.offset + position + length);
+    throw StringScannerException(message, span, string);
+  }
+}
+
+/// A class representing the state of a [SpanScanner].
+class _SpanScannerState implements LineScannerState {
+  /// The [SpanScanner] that created this.
+  final RelativeSpanScanner _scanner;
+
+  @override
+  final int position;
+  @override
+  int get line => _scanner._sourceFile.getLine(position);
+  @override
+  int get column => _scanner._sourceFile.getColumn(position);
+
+  _SpanScannerState(this._scanner, this.position);
+}
diff --git a/pkgs/string_scanner/lib/src/span_scanner.dart b/pkgs/string_scanner/lib/src/span_scanner.dart
new file mode 100644
index 0000000..509cf60
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/span_scanner.dart
@@ -0,0 +1,142 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:source_span/source_span.dart';
+
+import 'eager_span_scanner.dart';
+import 'exception.dart';
+import 'line_scanner.dart';
+import 'relative_span_scanner.dart';
+import 'string_scanner.dart';
+import 'utils.dart';
+
+/// A subclass of [LineScanner] that exposes matched ranges as source map
+/// [FileSpan]s.
+class SpanScanner extends StringScanner implements LineScanner {
+  /// The source of the scanner.
+  ///
+  /// This caches line break information and is used to generate [FileSpan]s.
+  final SourceFile _sourceFile;
+
+  @override
+  int get line => _sourceFile.getLine(position);
+  @override
+  int get column => _sourceFile.getColumn(position);
+
+  @override
+  LineScannerState get state => _SpanScannerState(this, position);
+
+  @override
+  set state(LineScannerState state) {
+    if (state is! _SpanScannerState || !identical(state._scanner, this)) {
+      throw ArgumentError('The given LineScannerState was not returned by '
+          'this LineScanner.');
+    }
+
+    position = state.position;
+  }
+
+  /// The [FileSpan] for [lastMatch].
+  ///
+  /// This is the span for the entire match. There's no way to get spans for
+  /// subgroups since [Match] exposes no information about their positions.
+  FileSpan? get lastSpan {
+    if (lastMatch == null) _lastSpan = null;
+    return _lastSpan;
+  }
+
+  FileSpan? _lastSpan;
+
+  /// The current location of the scanner.
+  FileLocation get location => _sourceFile.location(position);
+
+  /// Returns an empty span at the current location.
+  FileSpan get emptySpan => location.pointSpan();
+
+  /// Creates a new [SpanScanner] that starts scanning from [position].
+  ///
+  /// [sourceUrl] is used as [SourceLocation.sourceUrl] for the returned
+  /// [FileSpan]s as well as for error reporting. It can be a [String], a
+  /// [Uri], or `null`.
+  SpanScanner(super.string, {super.sourceUrl, super.position})
+      : _sourceFile = SourceFile.fromString(string, url: sourceUrl);
+
+  /// Creates a new [SpanScanner] that eagerly computes line and column numbers.
+  ///
+  /// In general [SpanScanner.new] will be more efficient, since it avoids extra
+  /// computation on every scan. However, eager scanning can be useful for
+  /// situations where the normal course of parsing frequently involves
+  /// accessing the current line and column numbers.
+  ///
+  /// Note that *only* the `line` and `column` fields on the `SpanScanner`
+  /// itself and its `LineScannerState` are eagerly computed. To limit their
+  /// memory footprint, returned spans and locations will still lazily compute
+  /// their line and column numbers.
+  factory SpanScanner.eager(String string, {sourceUrl, int? position}) =
+      EagerSpanScanner;
+
+  /// Creates a new [SpanScanner] that scans within [span].
+  ///
+  /// This scans through [span]`.text, but emits new spans from [span]`.file` in
+  /// their appropriate relative positions. The [string] field contains only
+  /// [span]`.text`, and [position], [line], and [column] are all relative to
+  /// the span.
+  factory SpanScanner.within(FileSpan span) = RelativeSpanScanner;
+
+  /// Creates a [FileSpan] representing the source range between [startState]
+  /// and the current position.
+  FileSpan spanFrom(LineScannerState startState, [LineScannerState? endState]) {
+    final endPosition = endState == null ? position : endState.position;
+    return _sourceFile.span(startState.position, endPosition);
+  }
+
+  /// Creates a [FileSpan] representing the source range between [startPosition]
+  /// and [endPosition], or the current position if [endPosition] is null.
+  ///
+  /// Each position should be a code unit offset into the string being scanned,
+  /// with the same conventions as [StringScanner.position].
+  ///
+  /// Throws a [RangeError] if [startPosition] or [endPosition] aren't within
+  /// this source file.
+  FileSpan spanFromPosition(int startPosition, [int? endPosition]) =>
+      _sourceFile.span(startPosition, endPosition ?? position);
+
+  @override
+  bool matches(Pattern pattern) {
+    if (!super.matches(pattern)) {
+      _lastSpan = null;
+      return false;
+    }
+
+    _lastSpan = _sourceFile.span(position, lastMatch!.end);
+    return true;
+  }
+
+  @override
+  Never error(String message, {Match? match, int? position, int? length}) {
+    validateErrorArgs(string, match, position, length);
+
+    if (match == null && position == null && length == null) match = lastMatch;
+    position ??= match == null ? this.position : match.start;
+    length ??= match == null ? 0 : match.end - match.start;
+
+    final span = _sourceFile.span(position, position + length);
+    throw StringScannerException(message, span, string);
+  }
+}
+
+/// A class representing the state of a [SpanScanner].
+class _SpanScannerState implements LineScannerState {
+  /// The [SpanScanner] that created this.
+  final SpanScanner _scanner;
+
+  @override
+  final int position;
+  @override
+  int get line => _scanner._sourceFile.getLine(position);
+  @override
+  int get column => _scanner._sourceFile.getColumn(position);
+
+  _SpanScannerState(this._scanner, this.position);
+}
diff --git a/pkgs/string_scanner/lib/src/string_scanner.dart b/pkgs/string_scanner/lib/src/string_scanner.dart
new file mode 100644
index 0000000..1466944
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/string_scanner.dart
@@ -0,0 +1,272 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:source_span/source_span.dart';
+
+import 'charcode.dart';
+import 'exception.dart';
+import 'utils.dart';
+
+/// A class that scans through a string using [Pattern]s.
+class StringScanner {
+  /// The URL of the source of the string being scanned.
+  ///
+  /// This is used for error reporting. It may be `null`, indicating that the
+  /// source URL is unknown or unavailable.
+  final Uri? sourceUrl;
+
+  /// The string being scanned through.
+  final String string;
+
+  /// The current position of the scanner in the string, in characters.
+  int get position => _position;
+  set position(int position) {
+    if (position.isNegative || position > string.length) {
+      throw ArgumentError('Invalid position $position');
+    }
+
+    _position = position;
+    _lastMatch = null;
+  }
+
+  int _position = 0;
+
+  /// The data about the previous match made by the scanner.
+  ///
+  /// If the last match failed, this will be `null`.
+  Match? get lastMatch {
+    // Lazily unset [_lastMatch] so that we avoid extra assignments in
+    // character-by-character methods that are used in core loops.
+    if (_position != _lastMatchPosition) _lastMatch = null;
+    return _lastMatch;
+  }
+
+  Match? _lastMatch;
+  int? _lastMatchPosition;
+
+  /// The portion of the string that hasn't yet been scanned.
+  String get rest => string.substring(position);
+
+  /// Whether the scanner has completely consumed [string].
+  bool get isDone => position == string.length;
+
+  /// Creates a new [StringScanner] that starts scanning from [position].
+  ///
+  /// [position] defaults to 0, the beginning of the string. [sourceUrl] is the
+  /// URL of the source of the string being scanned, if available. It can be
+  /// a [String], a [Uri], or `null`.
+  StringScanner(this.string, {Object? sourceUrl, int? position})
+      : sourceUrl = sourceUrl == null
+            ? null
+            : sourceUrl is String
+                ? Uri.parse(sourceUrl)
+                : sourceUrl as Uri {
+    if (position != null) this.position = position;
+  }
+
+  /// Consumes a single character and returns its character code.
+  ///
+  /// This throws a [FormatException] if the string has been fully consumed. It
+  /// doesn't affect [lastMatch].
+  int readChar() {
+    if (isDone) _fail('more input');
+    return string.codeUnitAt(_position++);
+  }
+
+  /// Returns the character code of the character [offset] away from [position].
+  ///
+  /// [offset] defaults to zero, and may be negative to inspect already-consumed
+  /// characters.
+  ///
+  /// This returns `null` if [offset] points outside the string. It doesn't
+  /// affect [lastMatch].
+  int? peekChar([int? offset]) {
+    offset ??= 0;
+    final index = position + offset;
+    if (index < 0 || index >= string.length) return null;
+    return string.codeUnitAt(index);
+  }
+
+  /// If the next character in the string is [character], consumes it.
+  ///
+  /// If [character] is a Unicode code point in a supplementary plane, this will
+  /// consume two code units. Dart's string representation is UTF-16, which
+  /// represents supplementary-plane code units as two code units.
+  ///
+  /// Returns whether or not [character] was consumed.
+  bool scanChar(int character) {
+    if (inSupplementaryPlane(character)) {
+      if (_position + 1 >= string.length ||
+          string.codeUnitAt(_position) != highSurrogate(character) ||
+          string.codeUnitAt(_position + 1) != lowSurrogate(character)) {
+        return false;
+      } else {
+        _position += 2;
+        return true;
+      }
+    } else {
+      if (isDone) return false;
+      if (string.codeUnitAt(_position) != character) return false;
+      _position++;
+      return true;
+    }
+  }
+
+  /// If the next character in the string is [character], consumes it.
+  ///
+  /// If [character] is a Unicode code point in a supplementary plane, this will
+  /// consume two code units. Dart's string representation is UTF-16, which
+  /// represents supplementary-plane code units as two code units.
+  ///
+  /// If [character] could not be consumed, throws a [FormatException]
+  /// describing the position of the failure. [name] is used in this error as
+  /// the expected name of the character being matched; if it's `null`, the
+  /// character itself is used instead.
+  void expectChar(int character, {String? name}) {
+    if (scanChar(character)) return;
+
+    if (name == null) {
+      if (character == $backslash) {
+        name = r'"\"';
+      } else if (character == $doubleQuote) {
+        name = r'"\""';
+      } else {
+        name = '"${String.fromCharCode(character)}"';
+      }
+    }
+
+    _fail(name);
+  }
+
+  /// Consumes a single Unicode code unit and returns it.
+  ///
+  /// This works like [readChar], except that it automatically handles UTF-16
+  /// surrogate pairs. Specifically, if the next two code units form a surrogate
+  /// pair, consumes them both and returns the corresponding Unicode code point.
+  ///
+  /// If next two characters are not a surrogate pair, the next code unit is
+  /// returned as-is, even if it's an unpaired surrogate.
+  int readCodePoint() {
+    final first = readChar();
+    if (!isHighSurrogate(first)) return first;
+
+    final next = peekChar();
+    if (next == null || !isLowSurrogate(next)) return first;
+
+    readChar();
+    return decodeSurrogatePair(first, next);
+  }
+
+  /// Returns the Unicode code point immediately after [position].
+  ///
+  /// This works like [peekChar], except that it automatically handles UTF-16
+  /// surrogate pairs. Specifically, if the next two code units form a surrogate
+  /// pair, returns the corresponding Unicode code point.
+  ///
+  /// If next two characters are not a surrogate pair, the next code unit is
+  /// returned as-is, even if it's an unpaired surrogate.
+  int? peekCodePoint() {
+    final first = peekChar();
+    if (first == null || !isHighSurrogate(first)) return first;
+
+    final next = peekChar(1);
+    if (next == null || !isLowSurrogate(next)) return first;
+
+    return decodeSurrogatePair(first, next);
+  }
+
+  /// If [pattern] matches at the current position of the string, scans forward
+  /// until the end of the match.
+  ///
+  /// Returns whether or not [pattern] matched.
+  bool scan(Pattern pattern) {
+    final success = matches(pattern);
+    if (success) {
+      _position = _lastMatch!.end;
+      _lastMatchPosition = _position;
+    }
+    return success;
+  }
+
+  /// If [pattern] matches at the current position of the string, scans forward
+  /// until the end of the match.
+  ///
+  /// If [pattern] did not match, throws a [FormatException] describing the
+  /// position of the failure. [name] is used in this error as the expected name
+  /// of the pattern being matched; if it's `null`, the pattern itself is used
+  /// instead.
+  void expect(Pattern pattern, {String? name}) {
+    if (scan(pattern)) return;
+
+    if (name == null) {
+      if (pattern is RegExp) {
+        final source = pattern.pattern;
+        name = '/$source/';
+      } else {
+        name =
+            pattern.toString().replaceAll(r'\', r'\\').replaceAll('"', r'\"');
+        name = '"$name"';
+      }
+    }
+    _fail(name);
+  }
+
+  /// If the string has not been fully consumed, this throws a
+  /// [FormatException].
+  void expectDone() {
+    if (isDone) return;
+    _fail('no more input');
+  }
+
+  /// Returns whether or not [pattern] matches at the current position of the
+  /// string.
+  ///
+  /// This doesn't move the scan pointer forward.
+  bool matches(Pattern pattern) {
+    _lastMatch = pattern.matchAsPrefix(string, position);
+    _lastMatchPosition = _position;
+    return _lastMatch != null;
+  }
+
+  /// Returns the substring of [string] between [start] and [end].
+  ///
+  /// Unlike [String.substring], [end] defaults to [position] rather than the
+  /// end of the string.
+  String substring(int start, [int? end]) {
+    end ??= position;
+    return string.substring(start, end);
+  }
+
+  /// Throws a [FormatException] with [message] as well as a detailed
+  /// description of the location of the error in the string.
+  ///
+  /// [match] is the match information for the span of the string with which the
+  /// error is associated. This should be a match returned by this scanner's
+  /// [lastMatch] property. By default, the error is associated with the last
+  /// match.
+  ///
+  /// If [position] and/or [length] are passed, they are used as the error span
+  /// instead. If only [length] is passed, [position] defaults to the current
+  /// position; if only [position] is passed, [length] defaults to 0.
+  ///
+  /// It's an error to pass [match] at the same time as [position] or [length].
+  Never error(String message, {Match? match, int? position, int? length}) {
+    validateErrorArgs(string, match, position, length);
+
+    if (match == null && position == null && length == null) match = lastMatch;
+    position ??= match == null ? this.position : match.start;
+    length ??= match == null ? 0 : match.end - match.start;
+
+    final sourceFile = SourceFile.fromString(string, url: sourceUrl);
+    final span = sourceFile.span(position, position + length);
+    throw StringScannerException(message, span, string);
+  }
+
+  // TODO(nweiz): Make this handle long lines more gracefully.
+  /// Throws a [FormatException] describing that [name] is expected at the
+  /// current position in the string.
+  Never _fail(String name) {
+    error('expected $name.', position: position, length: 0);
+  }
+}
diff --git a/pkgs/string_scanner/lib/src/utils.dart b/pkgs/string_scanner/lib/src/utils.dart
new file mode 100644
index 0000000..39891a1
--- /dev/null
+++ b/pkgs/string_scanner/lib/src/utils.dart
@@ -0,0 +1,95 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'string_scanner.dart';
+
+/// Validates the arguments passed to [StringScanner.error].
+void validateErrorArgs(
+    String string, Match? match, int? position, int? length) {
+  if (match != null && (position != null || length != null)) {
+    throw ArgumentError("Can't pass both match and position/length.");
+  }
+
+  if (position != null) {
+    if (position < 0) {
+      throw RangeError('position must be greater than or equal to 0.');
+    } else if (position > string.length) {
+      throw RangeError('position must be less than or equal to the '
+          'string length.');
+    }
+  }
+
+  if (length != null && length < 0) {
+    throw RangeError('length must be greater than or equal to 0.');
+  }
+
+  if (position != null && length != null && position + length > string.length) {
+    throw RangeError('position plus length must not go beyond the end of '
+        'the string.');
+  }
+}
+
+// See https://en.wikipedia.org/wiki/UTF-16#Code_points_from_U+010000_to_U+10FFFF
+// for documentation on how UTF-16 encoding works and definitions of various
+// related terms.
+
+/// The inclusive lower bound of Unicode's supplementary plane.
+const _supplementaryPlaneLowerBound = 0x10000;
+
+/// The inclusive upper bound of Unicode's supplementary plane.
+const _supplementaryPlaneUpperBound = 0x10FFFF;
+
+/// The inclusive lower bound of the UTF-16 high surrogate block.
+const _highSurrogateLowerBound = 0xD800;
+
+/// The inclusive lower bound of the UTF-16 low surrogate block.
+const _lowSurrogateLowerBound = 0xDC00;
+
+/// The number of low bits in each code unit of a surrogate pair that goes into
+/// determining which code point it encodes.
+const _surrogateBits = 10;
+
+/// A bit mask that covers the lower [_surrogateBits] of a code point, which can
+/// be used to extract the value of a surrogate or the low surrogate value of a
+/// code unit.
+const _surrogateValueMask = (1 << _surrogateBits) - 1;
+
+/// Returns whether [codePoint] is in the Unicode supplementary plane, and thus
+/// must be represented as a surrogate pair in UTF-16.
+bool inSupplementaryPlane(int codePoint) =>
+    codePoint >= _supplementaryPlaneLowerBound &&
+    codePoint <= _supplementaryPlaneUpperBound;
+
+/// Returns whether [codeUnit] is a UTF-16 high surrogate.
+bool isHighSurrogate(int codeUnit) =>
+    (codeUnit & ~_surrogateValueMask) == _highSurrogateLowerBound;
+
+/// Returns whether [codeUnit] is a UTF-16 low surrogate.
+bool isLowSurrogate(int codeUnit) =>
+    (codeUnit >> _surrogateBits) == (_lowSurrogateLowerBound >> _surrogateBits);
+
+/// Returns the high surrogate needed to encode the supplementary-plane
+/// [codePoint].
+int highSurrogate(int codePoint) {
+  assert(inSupplementaryPlane(codePoint));
+  return ((codePoint - _supplementaryPlaneLowerBound) >> _surrogateBits) +
+      _highSurrogateLowerBound;
+}
+
+/// Returns the low surrogate needed to encode the supplementary-plane
+/// [codePoint].
+int lowSurrogate(int codePoint) {
+  assert(inSupplementaryPlane(codePoint));
+  return ((codePoint - _supplementaryPlaneLowerBound) & _surrogateValueMask) +
+      _lowSurrogateLowerBound;
+}
+
+/// Converts a UTF-16 surrogate pair into the Unicode code unit it represents.
+int decodeSurrogatePair(int highSurrogate, int lowSurrogate) {
+  assert(isHighSurrogate(highSurrogate));
+  assert(isLowSurrogate(lowSurrogate));
+  return _supplementaryPlaneLowerBound +
+      (((highSurrogate & _surrogateValueMask) << _surrogateBits) |
+          (lowSurrogate & _surrogateValueMask));
+}
diff --git a/pkgs/string_scanner/lib/string_scanner.dart b/pkgs/string_scanner/lib/string_scanner.dart
new file mode 100644
index 0000000..e641ae7
--- /dev/null
+++ b/pkgs/string_scanner/lib/string_scanner.dart
@@ -0,0 +1,11 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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.
+
+/// A library for parsing strings using a sequence of patterns.
+library;
+
+export 'src/exception.dart';
+export 'src/line_scanner.dart';
+export 'src/span_scanner.dart';
+export 'src/string_scanner.dart';
diff --git a/pkgs/string_scanner/pubspec.yaml b/pkgs/string_scanner/pubspec.yaml
new file mode 100644
index 0000000..9b259cf
--- /dev/null
+++ b/pkgs/string_scanner/pubspec.yaml
@@ -0,0 +1,14 @@
+name: string_scanner
+version: 1.4.1
+description: A class for parsing strings using a sequence of patterns.
+repository: https://github.com/dart-lang/tools/tree/main/pkgs/string_scanner
+
+environment:
+  sdk: ^3.1.0
+
+dependencies:
+  source_span: ^1.8.0
+
+dev_dependencies:
+  dart_flutter_team_lints: ^3.0.0
+  test: ^1.16.6
diff --git a/pkgs/string_scanner/test/error_test.dart b/pkgs/string_scanner/test/error_test.dart
new file mode 100644
index 0000000..1f98c32
--- /dev/null
+++ b/pkgs/string_scanner/test/error_test.dart
@@ -0,0 +1,143 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:string_scanner/string_scanner.dart';
+import 'package:test/test.dart';
+
+import 'utils.dart';
+
+void main() {
+  test('defaults to the last match', () {
+    final scanner = StringScanner('foo bar baz');
+    scanner.expect('foo ');
+    scanner.expect('bar');
+    expect(() => scanner.error('oh no!'), throwsStringScannerException('bar'));
+  });
+
+  group('with match', () {
+    test('supports an earlier match', () {
+      final scanner = StringScanner('foo bar baz');
+      scanner.expect('foo ');
+      final match = scanner.lastMatch;
+      scanner.expect('bar');
+      expect(() => scanner.error('oh no!', match: match),
+          throwsStringScannerException('foo '));
+    });
+
+    test('supports a match on a previous line', () {
+      final scanner = StringScanner('foo bar baz\ndo re mi\nearth fire water');
+      scanner.expect('foo bar baz\ndo ');
+      scanner.expect('re');
+      final match = scanner.lastMatch;
+      scanner.expect(' mi\nearth ');
+      expect(() => scanner.error('oh no!', match: match),
+          throwsStringScannerException('re'));
+    });
+
+    test('supports a multiline match', () {
+      final scanner = StringScanner('foo bar baz\ndo re mi\nearth fire water');
+      scanner.expect('foo bar ');
+      scanner.expect('baz\ndo');
+      final match = scanner.lastMatch;
+      scanner.expect(' re mi');
+      expect(() => scanner.error('oh no!', match: match),
+          throwsStringScannerException('baz\ndo'));
+    });
+
+    test('supports a match after position', () {
+      final scanner = StringScanner('foo bar baz');
+      scanner.expect('foo ');
+      scanner.expect('bar');
+      final match = scanner.lastMatch;
+      scanner.position = 0;
+      expect(() => scanner.error('oh no!', match: match),
+          throwsStringScannerException('bar'));
+    });
+  });
+
+  group('with position and/or length', () {
+    test('defaults to length 0', () {
+      final scanner = StringScanner('foo bar baz');
+      scanner.expect('foo ');
+      expect(() => scanner.error('oh no!', position: 1),
+          throwsStringScannerException(''));
+    });
+
+    test('defaults to the current position', () {
+      final scanner = StringScanner('foo bar baz');
+      scanner.expect('foo ');
+      expect(() => scanner.error('oh no!', length: 3),
+          throwsStringScannerException('bar'));
+    });
+
+    test('supports an earlier position', () {
+      final scanner = StringScanner('foo bar baz');
+      scanner.expect('foo ');
+      expect(() => scanner.error('oh no!', position: 1, length: 2),
+          throwsStringScannerException('oo'));
+    });
+
+    test('supports a position on a previous line', () {
+      final scanner = StringScanner('foo bar baz\ndo re mi\nearth fire water');
+      scanner.expect('foo bar baz\ndo re mi\nearth');
+      expect(() => scanner.error('oh no!', position: 15, length: 2),
+          throwsStringScannerException('re'));
+    });
+
+    test('supports a multiline length', () {
+      final scanner = StringScanner('foo bar baz\ndo re mi\nearth fire water');
+      scanner.expect('foo bar baz\ndo re mi\nearth');
+      expect(() => scanner.error('oh no!', position: 8, length: 8),
+          throwsStringScannerException('baz\ndo r'));
+    });
+
+    test('supports a position after the current one', () {
+      final scanner = StringScanner('foo bar baz');
+      expect(() => scanner.error('oh no!', position: 4, length: 3),
+          throwsStringScannerException('bar'));
+    });
+
+    test('supports a length of zero', () {
+      final scanner = StringScanner('foo bar baz');
+      expect(() => scanner.error('oh no!', position: 4, length: 0),
+          throwsStringScannerException(''));
+    });
+  });
+
+  group('argument errors', () {
+    late StringScanner scanner;
+    setUp(() {
+      scanner = StringScanner('foo bar baz');
+      scanner.scan('foo');
+    });
+
+    test('if match is passed with position', () {
+      expect(
+          () => scanner.error('oh no!', match: scanner.lastMatch, position: 1),
+          throwsArgumentError);
+    });
+
+    test('if match is passed with length', () {
+      expect(() => scanner.error('oh no!', match: scanner.lastMatch, length: 1),
+          throwsArgumentError);
+    });
+
+    test('if position is negative', () {
+      expect(() => scanner.error('oh no!', position: -1), throwsArgumentError);
+    });
+
+    test('if position is outside the string', () {
+      expect(() => scanner.error('oh no!', position: 100), throwsArgumentError);
+    });
+
+    test('if position + length is outside the string', () {
+      expect(() => scanner.error('oh no!', position: 7, length: 7),
+          throwsArgumentError);
+    });
+
+    test('if length is negative', () {
+      expect(() => scanner.error('oh no!', length: -1), throwsArgumentError);
+    });
+  });
+}
diff --git a/pkgs/string_scanner/test/line_scanner_test.dart b/pkgs/string_scanner/test/line_scanner_test.dart
new file mode 100644
index 0000000..1af5c36
--- /dev/null
+++ b/pkgs/string_scanner/test/line_scanner_test.dart
@@ -0,0 +1,465 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:string_scanner/src/charcode.dart';
+import 'package:string_scanner/string_scanner.dart';
+import 'package:test/test.dart';
+
+void main() {
+  late LineScanner scanner;
+  setUp(() {
+    scanner = LineScanner('foo\nbar\r\nbaz');
+  });
+
+  test('begins with line and column 0', () {
+    expect(scanner.line, equals(0));
+    expect(scanner.column, equals(0));
+  });
+
+  group('scan()', () {
+    test('consuming no newlines increases the column but not the line', () {
+      scanner.expect('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+    });
+
+    test('consuming a LF resets the column and increases the line', () {
+      scanner.expect('foo\nba');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(2));
+    });
+
+    test('consuming multiple LFs resets the column and increases the line', () {
+      scanner.expect('foo\nbar\r\nb');
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test('consuming a CR LF increases the line only after the LF', () {
+      scanner.expect('foo\nbar\r');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+
+      scanner.expect('\nb');
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test('consuming a CR not followed by LF increases the line', () {
+      scanner = LineScanner('foo\nbar\rbaz');
+      scanner.expect('foo\nbar\r');
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+
+      scanner.expect('b');
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test('consuming a CR at the end increases the line', () {
+      scanner = LineScanner('foo\nbar\r');
+      scanner.expect('foo\nbar\r');
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+      expect(scanner.isDone, isTrue);
+    });
+
+    test('consuming a mix of CR, LF, CR+LF increases the line', () {
+      scanner = LineScanner('0\n1\r2\r\n3');
+      scanner.expect('0\n1\r2\r\n3');
+      expect(scanner.line, equals(3));
+      expect(scanner.column, equals(1));
+    });
+
+    test('scanning a zero length match between CR LF does not fail', () {
+      scanner.expect('foo\nbar\r');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+      scanner.expect(RegExp('(?!x)'));
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+    });
+  });
+
+  group('readChar()', () {
+    test('on a non-newline character increases the column but not the line',
+        () {
+      scanner.readChar();
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(1));
+    });
+
+    test('consuming a LF resets the column and increases the line', () {
+      scanner.expect('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+
+      scanner.readChar();
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a CR LF increases the line only after the LF', () {
+      scanner = LineScanner('foo\r\nbar');
+      scanner.expect('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+
+      scanner.readChar();
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(4));
+
+      scanner.readChar();
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a CR not followed by a LF increases the line', () {
+      scanner = LineScanner('foo\nbar\rbaz');
+      scanner.expect('foo\nbar');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(3));
+
+      scanner.readChar();
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a CR at the end increases the line', () {
+      scanner = LineScanner('foo\nbar\r');
+      scanner.expect('foo\nbar');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(3));
+
+      scanner.readChar();
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a mix of CR, LF, CR+LF increases the line', () {
+      scanner = LineScanner('0\n1\r2\r\n3');
+      for (var i = 0; i < scanner.string.length; i++) {
+        scanner.readChar();
+      }
+
+      expect(scanner.line, equals(3));
+      expect(scanner.column, equals(1));
+    });
+  });
+
+  group('readCodePoint()', () {
+    test('on a non-newline character increases the column but not the line',
+        () {
+      scanner.readCodePoint();
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(1));
+    });
+
+    test('consuming a newline resets the column and increases the line', () {
+      scanner.expect('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+
+      scanner.readCodePoint();
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(0));
+    });
+
+    test("consuming halfway through a CR LF doesn't count as a line", () {
+      scanner.expect('foo\nbar');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(3));
+
+      scanner.readCodePoint();
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+
+      scanner.readCodePoint();
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+    });
+  });
+
+  group('scanChar()', () {
+    test('on a non-newline character increases the column but not the line',
+        () {
+      scanner.scanChar($f);
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(1));
+    });
+
+    test('consuming a LF resets the column and increases the line', () {
+      scanner.expect('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+
+      scanner.scanChar($lf);
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a CR LF increases the line only after the LF', () {
+      scanner.expect('foo\nbar');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(3));
+
+      scanner.scanChar($cr);
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+
+      scanner.scanChar($lf);
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a CR not followed by LF increases the line', () {
+      scanner = LineScanner('foo\rbar');
+      scanner.expect('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+
+      scanner.scanChar($cr);
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a CR at the end increases the line', () {
+      scanner = LineScanner('foo\r');
+      scanner.expect('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+
+      scanner.scanChar($cr);
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(0));
+    });
+
+    test('consuming a mix of CR, LF, CR+LF increases the line', () {
+      scanner = LineScanner('0\n1\r2\r\n3');
+      for (var i = 0; i < scanner.string.length; i++) {
+        scanner.scanChar(scanner.string[i].codeUnits.single);
+      }
+
+      expect(scanner.line, equals(3));
+      expect(scanner.column, equals(1));
+    });
+  });
+
+  group('before a surrogate pair', () {
+    final codePoint = '\uD83D\uDC6D'.runes.first;
+    const highSurrogate = 0xD83D;
+
+    late LineScanner scanner;
+    setUp(() {
+      scanner = LineScanner('foo: \uD83D\uDC6D');
+      expect(scanner.scan('foo: '), isTrue);
+    });
+
+    test('readChar returns the high surrogate and moves into the pair', () {
+      expect(scanner.readChar(), equals(highSurrogate));
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(6));
+      expect(scanner.position, equals(6));
+    });
+
+    test('readCodePoint returns the code unit and moves past the pair', () {
+      expect(scanner.readCodePoint(), equals(codePoint));
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(7));
+      expect(scanner.position, equals(7));
+    });
+
+    test('scanChar with the high surrogate moves into the pair', () {
+      expect(scanner.scanChar(highSurrogate), isTrue);
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(6));
+      expect(scanner.position, equals(6));
+    });
+
+    test('scanChar with the code point moves past the pair', () {
+      expect(scanner.scanChar(codePoint), isTrue);
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(7));
+      expect(scanner.position, equals(7));
+    });
+
+    test('expectChar with the high surrogate moves into the pair', () {
+      scanner.expectChar(highSurrogate);
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(6));
+      expect(scanner.position, equals(6));
+    });
+
+    test('expectChar with the code point moves past the pair', () {
+      scanner.expectChar(codePoint);
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(7));
+      expect(scanner.position, equals(7));
+    });
+  });
+
+  group('position=', () {
+    test('forward through LFs sets the line and column', () {
+      scanner = LineScanner('foo\nbar\nbaz');
+      scanner.position = 9; // "foo\nbar\nb"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test('forward from non-zero character through LFs sets the line and column',
+        () {
+      scanner = LineScanner('foo\nbar\nbaz');
+      scanner.expect('fo');
+      scanner.position = 9; // "foo\nbar\nb"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test('forward through CR LFs sets the line and column', () {
+      scanner = LineScanner('foo\r\nbar\r\nbaz');
+      scanner.position = 11; // "foo\r\nbar\r\nb"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test('forward through CR not followed by LFs sets the line and column', () {
+      scanner = LineScanner('foo\rbar\rbaz');
+      scanner.position = 9; // "foo\rbar\rb"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test('forward through CR at end sets the line and column', () {
+      scanner = LineScanner('foo\rbar\r');
+      scanner.position = 8; // "foo\rbar\r"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+    });
+
+    test('forward through a mix of CR, LF, CR+LF sets the line and column', () {
+      scanner = LineScanner('0\n1\r2\r\n3');
+      scanner.position = scanner.string.length;
+
+      expect(scanner.line, equals(3));
+      expect(scanner.column, equals(1));
+    });
+
+    test('forward through no newlines sets the column', () {
+      scanner.position = 2; // "fo"
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(2));
+    });
+
+    test('backward through LFs sets the line and column', () {
+      scanner = LineScanner('foo\nbar\nbaz');
+      scanner.expect('foo\nbar\nbaz');
+      scanner.position = 2; // "fo"
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(2));
+    });
+
+    test('backward through CR LFs sets the line and column', () {
+      scanner = LineScanner('foo\r\nbar\r\nbaz');
+      scanner.expect('foo\r\nbar\r\nbaz');
+      scanner.position = 2; // "fo"
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(2));
+    });
+
+    test('backward through CR not followed by LFs sets the line and column',
+        () {
+      scanner = LineScanner('foo\rbar\rbaz');
+      scanner.expect('foo\rbar\rbaz');
+      scanner.position = 2; // "fo"
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(2));
+    });
+
+    test('backward through CR at end sets the line and column', () {
+      scanner = LineScanner('foo\rbar\r');
+      scanner.expect('foo\rbar\r');
+      scanner.position = 2; // "fo"
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(2));
+    });
+
+    test('backward through a mix of CR, LF, CR+LF sets the line and column',
+        () {
+      scanner = LineScanner('0\n1\r2\r\n3');
+      scanner.expect(scanner.string);
+
+      scanner.position = 1;
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(1));
+    });
+
+    test('backward through no newlines sets the column', () {
+      scanner.expect('foo\nbar\r\nbaz');
+      scanner.position = 10; // "foo\nbar\r\nb"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(1));
+    });
+
+    test("forward halfway through a CR LF doesn't count as a line", () {
+      scanner.position = 8; // "foo\nbar\r"
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+    });
+
+    test('forward from halfway through a CR LF counts as a line', () {
+      scanner.expect('foo\nbar\r');
+      scanner.position = 11; // "foo\nbar\r\nba"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(2));
+    });
+
+    test('backward to between CR LF', () {
+      scanner.expect('foo\nbar\r\nbaz');
+      scanner.position = 8; // "foo\nbar\r"
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+    });
+
+    test('backward from between CR LF', () {
+      scanner.expect('foo\nbar\r');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(4));
+      scanner.position = 5; // "foo\nb"
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(1));
+    });
+
+    test('backward to after CR LF', () {
+      scanner.expect('foo\nbar\r\nbaz');
+      scanner.position = 9; // "foo\nbar\r\n"
+      expect(scanner.line, equals(2));
+      expect(scanner.column, equals(0));
+    });
+
+    test('backward to before CR LF', () {
+      scanner.expect('foo\nbar\r\nbaz');
+      scanner.position = 7; // "foo\nbar"
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(3));
+    });
+  });
+
+  test('state= restores the line, column, and position', () {
+    scanner.expect('foo\nb');
+    final state = scanner.state;
+
+    scanner.scan('ar\nba');
+    scanner.state = state;
+    expect(scanner.rest, equals('ar\r\nbaz'));
+    expect(scanner.line, equals(1));
+    expect(scanner.column, equals(1));
+  });
+
+  test('state= rejects a foreign state', () {
+    scanner.scan('foo\nb');
+
+    expect(() => LineScanner(scanner.string).state = scanner.state,
+        throwsArgumentError);
+  });
+}
diff --git a/pkgs/string_scanner/test/span_scanner_test.dart b/pkgs/string_scanner/test/span_scanner_test.dart
new file mode 100644
index 0000000..93d9c47
--- /dev/null
+++ b/pkgs/string_scanner/test/span_scanner_test.dart
@@ -0,0 +1,238 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:source_span/source_span.dart';
+import 'package:string_scanner/string_scanner.dart';
+import 'package:test/test.dart';
+
+import 'utils.dart';
+
+void main() {
+  testForImplementation(
+      'lazy',
+      ([String? string]) =>
+          SpanScanner(string ?? 'foo\nbar\nbaz', sourceUrl: 'source'));
+
+  testForImplementation(
+      'eager',
+      ([String? string]) =>
+          SpanScanner.eager(string ?? 'foo\nbar\nbaz', sourceUrl: 'source'));
+
+  group('within', () {
+    const text = 'first\nbefore: foo\nbar\nbaz :after\nlast';
+    final startOffset = text.indexOf('foo');
+
+    late SpanScanner scanner;
+    setUp(() {
+      final file = SourceFile.fromString(text, url: 'source');
+      scanner =
+          SpanScanner.within(file.span(startOffset, text.indexOf(' :after')));
+    });
+
+    test('string only includes the span text', () {
+      expect(scanner.string, equals('foo\nbar\nbaz'));
+    });
+
+    test('line and column are span-relative', () {
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(0));
+
+      scanner.scan('foo');
+      expect(scanner.line, equals(0));
+      expect(scanner.column, equals(3));
+
+      scanner.scan('\n');
+      expect(scanner.line, equals(1));
+      expect(scanner.column, equals(0));
+    });
+
+    test('tracks the span for the last match', () {
+      scanner.scan('fo');
+      scanner.scan('o\nba');
+
+      final span = scanner.lastSpan!;
+      expect(span.start.offset, equals(startOffset + 2));
+      expect(span.start.line, equals(1));
+      expect(span.start.column, equals(10));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.end.offset, equals(startOffset + 6));
+      expect(span.end.line, equals(2));
+      expect(span.end.column, equals(2));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.text, equals('o\nba'));
+    });
+
+    test('.spanFrom() returns a span from a previous state', () {
+      scanner.scan('fo');
+      final state = scanner.state;
+      scanner.scan('o\nba');
+      scanner.scan('r\nba');
+
+      final span = scanner.spanFrom(state);
+      expect(span.text, equals('o\nbar\nba'));
+    });
+
+    test('.spanFromPosition() returns a span from a previous state', () {
+      scanner.scan('fo');
+      final start = scanner.position;
+      scanner.scan('o\nba');
+      scanner.scan('r\nba');
+
+      final span = scanner.spanFromPosition(start + 2, start + 5);
+      expect(span.text, equals('bar'));
+    });
+
+    test('.emptySpan returns an empty span at the current location', () {
+      scanner.scan('foo\nba');
+
+      final span = scanner.emptySpan;
+      expect(span.start.offset, equals(startOffset + 6));
+      expect(span.start.line, equals(2));
+      expect(span.start.column, equals(2));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.end.offset, equals(startOffset + 6));
+      expect(span.end.line, equals(2));
+      expect(span.end.column, equals(2));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.text, equals(''));
+    });
+
+    test('.error() uses an absolute span', () {
+      scanner.expect('foo');
+      expect(
+          () => scanner.error('oh no!'), throwsStringScannerException('foo'));
+    });
+
+    test('.isDone returns true at the end of the span', () {
+      scanner.expect('foo\nbar\nbaz');
+      expect(scanner.isDone, isTrue);
+    });
+  });
+}
+
+void testForImplementation(
+    String name, SpanScanner Function([String string]) create) {
+  group('for a $name scanner', () {
+    late SpanScanner scanner;
+    setUp(() => scanner = create());
+
+    test('tracks the span for the last match', () {
+      scanner.scan('fo');
+      scanner.scan('o\nba');
+
+      final span = scanner.lastSpan!;
+      expect(span.start.offset, equals(2));
+      expect(span.start.line, equals(0));
+      expect(span.start.column, equals(2));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.end.offset, equals(6));
+      expect(span.end.line, equals(1));
+      expect(span.end.column, equals(2));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.text, equals('o\nba'));
+    });
+
+    test('.spanFrom() returns a span from a previous state', () {
+      scanner.scan('fo');
+      final state = scanner.state;
+      scanner.scan('o\nba');
+      scanner.scan('r\nba');
+
+      final span = scanner.spanFrom(state);
+      expect(span.text, equals('o\nbar\nba'));
+    });
+
+    test('.spanFromPosition() returns a span from a previous state', () {
+      scanner.scan('fo');
+      final start = scanner.position;
+      scanner.scan('o\nba');
+      scanner.scan('r\nba');
+
+      final span = scanner.spanFromPosition(start + 2, start + 5);
+      expect(span.text, equals('bar'));
+    });
+
+    test('.emptySpan returns an empty span at the current location', () {
+      scanner.scan('foo\nba');
+
+      final span = scanner.emptySpan;
+      expect(span.start.offset, equals(6));
+      expect(span.start.line, equals(1));
+      expect(span.start.column, equals(2));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.end.offset, equals(6));
+      expect(span.end.line, equals(1));
+      expect(span.end.column, equals(2));
+      expect(span.start.sourceUrl, equals(Uri.parse('source')));
+
+      expect(span.text, equals(''));
+    });
+
+    group('before a surrogate pair', () {
+      final codePoint = '\uD83D\uDC6D'.runes.first;
+      const highSurrogate = 0xD83D;
+
+      late SpanScanner scanner;
+      setUp(() {
+        scanner = create('foo: \uD83D\uDC6D bar');
+        expect(scanner.scan('foo: '), isTrue);
+      });
+
+      test('readChar returns the high surrogate and moves into the pair', () {
+        expect(scanner.readChar(), equals(highSurrogate));
+        expect(scanner.line, equals(0));
+        expect(scanner.column, equals(6));
+        expect(scanner.position, equals(6));
+      });
+
+      test('readCodePoint returns the code unit and moves past the pair', () {
+        expect(scanner.readCodePoint(), equals(codePoint));
+        expect(scanner.line, equals(0));
+        expect(scanner.column, equals(7));
+        expect(scanner.position, equals(7));
+      });
+
+      test('scanChar with the high surrogate moves into the pair', () {
+        expect(scanner.scanChar(highSurrogate), isTrue);
+        expect(scanner.line, equals(0));
+        expect(scanner.column, equals(6));
+        expect(scanner.position, equals(6));
+      });
+
+      test('scanChar with the code point moves past the pair', () {
+        expect(scanner.scanChar(codePoint), isTrue);
+        expect(scanner.line, equals(0));
+        expect(scanner.column, equals(7));
+        expect(scanner.position, equals(7));
+      });
+
+      test('expectChar with the high surrogate moves into the pair', () {
+        scanner.expectChar(highSurrogate);
+        expect(scanner.line, equals(0));
+        expect(scanner.column, equals(6));
+        expect(scanner.position, equals(6));
+      });
+
+      test('expectChar with the code point moves past the pair', () {
+        scanner.expectChar(codePoint);
+        expect(scanner.line, equals(0));
+        expect(scanner.column, equals(7));
+        expect(scanner.position, equals(7));
+      });
+
+      test('spanFrom covers the surrogate pair', () {
+        final state = scanner.state;
+        scanner.scan('\uD83D\uDC6D b');
+        expect(scanner.spanFrom(state).text, equals('\uD83D\uDC6D b'));
+      });
+    });
+  });
+}
diff --git a/pkgs/string_scanner/test/string_scanner_test.dart b/pkgs/string_scanner/test/string_scanner_test.dart
new file mode 100644
index 0000000..36a737e
--- /dev/null
+++ b/pkgs/string_scanner/test/string_scanner_test.dart
@@ -0,0 +1,564 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:string_scanner/src/charcode.dart';
+import 'package:string_scanner/string_scanner.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('with an empty string', () {
+    late StringScanner scanner;
+    setUp(() {
+      scanner = StringScanner('');
+    });
+
+    test('is done', () {
+      expect(scanner.isDone, isTrue);
+      expect(scanner.expectDone, isNot(throwsFormatException));
+    });
+
+    test('rest is empty', () {
+      expect(scanner.rest, isEmpty);
+    });
+
+    test('lastMatch is null', () {
+      expect(scanner.lastMatch, isNull);
+    });
+
+    test('position is zero', () {
+      expect(scanner.position, equals(0));
+    });
+
+    test("readChar fails and doesn't change the state", () {
+      expect(scanner.readChar, throwsFormatException);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("readCodePoint fails and doesn't change the state", () {
+      expect(scanner.readCodePoint, throwsFormatException);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("peekChar returns null and doesn't change the state", () {
+      expect(scanner.peekChar(), isNull);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("peekCodePoint returns null and doesn't change the state", () {
+      expect(scanner.peekCodePoint(), isNull);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("scanChar returns false and doesn't change the state", () {
+      expect(scanner.scanChar($f), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("expectChar fails and doesn't change the state", () {
+      expect(() => scanner.expectChar($f), throwsFormatException);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("scan returns false and doesn't change the state", () {
+      expect(scanner.scan(RegExp('.')), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("expect throws a FormatException and doesn't change the state", () {
+      expect(() => scanner.expect(RegExp('.')), throwsFormatException);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test("matches returns false and doesn't change the state", () {
+      expect(scanner.matches(RegExp('.')), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test('substring returns the empty string', () {
+      expect(scanner.substring(0), isEmpty);
+    });
+
+    test('setting position to 1 throws an ArgumentError', () {
+      expect(() {
+        scanner.position = 1;
+      }, throwsArgumentError);
+    });
+
+    test('setting position to -1 throws an ArgumentError', () {
+      expect(() {
+        scanner.position = -1;
+      }, throwsArgumentError);
+    });
+  });
+
+  group('at the beginning of a string', () {
+    late StringScanner scanner;
+    setUp(() {
+      scanner = StringScanner('foo bar');
+    });
+
+    test('is not done', () {
+      expect(scanner.isDone, isFalse);
+      expect(scanner.expectDone, throwsFormatException);
+    });
+
+    test('rest is the whole string', () {
+      expect(scanner.rest, equals('foo bar'));
+    });
+
+    test('lastMatch is null', () {
+      expect(scanner.lastMatch, isNull);
+    });
+
+    test('position is zero', () {
+      expect(scanner.position, equals(0));
+    });
+
+    test('readChar returns the first character and moves forward', () {
+      expect(scanner.readChar(), equals(0x66));
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(1));
+    });
+
+    test('readCodePoint returns the first character and moves forward', () {
+      expect(scanner.readCodePoint(), equals(0x66));
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(1));
+    });
+
+    test('peekChar returns the first character', () {
+      expect(scanner.peekChar(), equals(0x66));
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test('peekChar with an argument returns the nth character', () {
+      expect(scanner.peekChar(4), equals(0x62));
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test('peekCodePoint returns the first character', () {
+      expect(scanner.peekCodePoint(), equals(0x66));
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test('a matching scanChar returns true moves forward', () {
+      expect(scanner.scanChar($f), isTrue);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(1));
+    });
+
+    test('a non-matching scanChar returns false and does nothing', () {
+      expect(scanner.scanChar($x), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test('a matching expectChar moves forward', () {
+      scanner.expectChar($f);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(1));
+    });
+
+    test('a non-matching expectChar fails', () {
+      expect(() => scanner.expectChar($x), throwsFormatException);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+    });
+
+    test('a matching scan returns true and changes the state', () {
+      expect(scanner.scan(RegExp('f(..)')), isTrue);
+      expect(scanner.lastMatch![1], equals('oo'));
+      expect(scanner.position, equals(3));
+      expect(scanner.rest, equals(' bar'));
+    });
+
+    test('a non-matching scan returns false and sets lastMatch to null', () {
+      expect(scanner.matches(RegExp('f(..)')), isTrue);
+      expect(scanner.lastMatch, isNotNull);
+
+      expect(scanner.scan(RegExp('b(..)')), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+      expect(scanner.rest, equals('foo bar'));
+    });
+
+    test('a matching expect changes the state', () {
+      scanner.expect(RegExp('f(..)'));
+      expect(scanner.lastMatch![1], equals('oo'));
+      expect(scanner.position, equals(3));
+      expect(scanner.rest, equals(' bar'));
+    });
+
+    test(
+        'a non-matching expect throws a FormatException and sets lastMatch to '
+        'null', () {
+      expect(scanner.matches(RegExp('f(..)')), isTrue);
+      expect(scanner.lastMatch, isNotNull);
+
+      expect(() => scanner.expect(RegExp('b(..)')), throwsFormatException);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+      expect(scanner.rest, equals('foo bar'));
+    });
+
+    test('a matching matches returns true and only changes lastMatch', () {
+      expect(scanner.matches(RegExp('f(..)')), isTrue);
+      expect(scanner.lastMatch![1], equals('oo'));
+      expect(scanner.position, equals(0));
+      expect(scanner.rest, equals('foo bar'));
+    });
+
+    test("a non-matching matches returns false and doesn't change the state",
+        () {
+      expect(scanner.matches(RegExp('b(..)')), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(0));
+      expect(scanner.rest, equals('foo bar'));
+    });
+
+    test('substring from the beginning returns the empty string', () {
+      expect(scanner.substring(0), isEmpty);
+    });
+
+    test('substring with a custom end returns the substring', () {
+      expect(scanner.substring(0, 3), equals('foo'));
+    });
+
+    test('substring with the string length returns the whole string', () {
+      expect(scanner.substring(0, 7), equals('foo bar'));
+    });
+
+    test('setting position to 1 moves the cursor forward', () {
+      scanner.position = 1;
+      expect(scanner.position, equals(1));
+      expect(scanner.rest, equals('oo bar'));
+
+      expect(scanner.scan(RegExp('oo.')), isTrue);
+      expect(scanner.lastMatch![0], equals('oo '));
+      expect(scanner.position, equals(4));
+      expect(scanner.rest, equals('bar'));
+    });
+
+    test('setting position beyond the string throws an ArgumentError', () {
+      expect(() {
+        scanner.position = 8;
+      }, throwsArgumentError);
+    });
+
+    test('setting position to -1 throws an ArgumentError', () {
+      expect(() {
+        scanner.position = -1;
+      }, throwsArgumentError);
+    });
+
+    test('scan accepts any Pattern', () {
+      expect(scanner.scan('foo'), isTrue);
+      expect(scanner.lastMatch![0], equals('foo'));
+      expect(scanner.position, equals(3));
+      expect(scanner.rest, equals(' bar'));
+    });
+
+    test('scans multiple times', () {
+      expect(scanner.scan(RegExp('f(..)')), isTrue);
+      expect(scanner.lastMatch![1], equals('oo'));
+      expect(scanner.position, equals(3));
+      expect(scanner.rest, equals(' bar'));
+
+      expect(scanner.scan(RegExp(' b(..)')), isTrue);
+      expect(scanner.lastMatch![1], equals('ar'));
+      expect(scanner.position, equals(7));
+      expect(scanner.rest, equals(''));
+      expect(scanner.isDone, isTrue);
+      expect(scanner.expectDone, isNot(throwsFormatException));
+    });
+  });
+
+  group('after a scan', () {
+    late StringScanner scanner;
+    setUp(() {
+      scanner = StringScanner('foo bar');
+      expect(scanner.scan('foo'), isTrue);
+    });
+
+    test('readChar returns the first character and unsets the last match', () {
+      expect(scanner.readChar(), equals($space));
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(4));
+    });
+
+    test('readCodePoint returns the first character and unsets the last match',
+        () {
+      expect(scanner.readCodePoint(), equals($space));
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(4));
+    });
+
+    test('a matching scanChar returns true and unsets the last match', () {
+      expect(scanner.scanChar($space), isTrue);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(4));
+    });
+
+    test('a matching expectChar returns true and unsets the last match', () {
+      scanner.expectChar($space);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(4));
+    });
+  });
+
+  group('at the end of a string', () {
+    late StringScanner scanner;
+    setUp(() {
+      scanner = StringScanner('foo bar');
+      expect(scanner.scan('foo bar'), isTrue);
+    });
+
+    test('is done', () {
+      expect(scanner.isDone, isTrue);
+      expect(scanner.expectDone, isNot(throwsFormatException));
+    });
+
+    test('rest is empty', () {
+      expect(scanner.rest, isEmpty);
+    });
+
+    test('position is zero', () {
+      expect(scanner.position, equals(7));
+    });
+
+    test("readChar fails and doesn't change the state", () {
+      expect(scanner.readChar, throwsFormatException);
+      expect(scanner.lastMatch, isNotNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test("readCodePoint fails and doesn't change the state", () {
+      expect(scanner.readCodePoint, throwsFormatException);
+      expect(scanner.lastMatch, isNotNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test("peekChar returns null and doesn't change the state", () {
+      expect(scanner.peekChar(), isNull);
+      expect(scanner.lastMatch, isNotNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test("peekCodePoint returns null and doesn't change the state", () {
+      expect(scanner.peekCodePoint(), isNull);
+      expect(scanner.lastMatch, isNotNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test("scanChar returns false and doesn't change the state", () {
+      expect(scanner.scanChar($f), isFalse);
+      expect(scanner.lastMatch, isNotNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test("expectChar fails and doesn't change the state", () {
+      expect(() => scanner.expectChar($f), throwsFormatException);
+      expect(scanner.lastMatch, isNotNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test('scan returns false and sets lastMatch to null', () {
+      expect(scanner.scan(RegExp('.')), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test('expect throws a FormatException and sets lastMatch to null', () {
+      expect(() => scanner.expect(RegExp('.')), throwsFormatException);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test('matches returns false sets lastMatch to null', () {
+      expect(scanner.matches(RegExp('.')), isFalse);
+      expect(scanner.lastMatch, isNull);
+      expect(scanner.position, equals(7));
+    });
+
+    test('substring from the beginning returns the whole string', () {
+      expect(scanner.substring(0), equals('foo bar'));
+    });
+
+    test('substring with a custom start returns a substring from there', () {
+      expect(scanner.substring(4), equals('bar'));
+    });
+
+    test('substring with a custom start and end returns that substring', () {
+      expect(scanner.substring(3, 5), equals(' b'));
+    });
+
+    test('setting position to 1 moves the cursor backward', () {
+      scanner.position = 1;
+      expect(scanner.position, equals(1));
+      expect(scanner.rest, equals('oo bar'));
+
+      expect(scanner.scan(RegExp('oo.')), isTrue);
+      expect(scanner.lastMatch![0], equals('oo '));
+      expect(scanner.position, equals(4));
+      expect(scanner.rest, equals('bar'));
+    });
+
+    test('setting and resetting position clears lastMatch', () {
+      final oldPosition = scanner.position;
+      scanner.position = 1;
+      scanner.position = oldPosition;
+      expect(scanner.lastMatch, isNull);
+    });
+
+    test('setting position beyond the string throws an ArgumentError', () {
+      expect(() {
+        scanner.position = 8;
+      }, throwsArgumentError);
+    });
+
+    test('setting position to -1 throws an ArgumentError', () {
+      expect(() {
+        scanner.position = -1;
+      }, throwsArgumentError);
+    });
+  });
+
+  group('before a surrogate pair', () {
+    final codePoint = '\uD83D\uDC6D'.runes.first;
+    const highSurrogate = 0xD83D;
+
+    late StringScanner scanner;
+    setUp(() {
+      scanner = StringScanner('foo: \uD83D\uDC6D');
+      expect(scanner.scan('foo: '), isTrue);
+    });
+
+    test('readChar returns the high surrogate and moves into the pair', () {
+      expect(scanner.readChar(), equals(highSurrogate));
+      expect(scanner.position, equals(6));
+    });
+
+    test('readCodePoint returns the code unit and moves past the pair', () {
+      expect(scanner.readCodePoint(), equals(codePoint));
+      expect(scanner.position, equals(7));
+    });
+
+    test('peekChar returns the high surrogate', () {
+      expect(scanner.peekChar(), equals(highSurrogate));
+      expect(scanner.position, equals(5));
+    });
+
+    test('peekCodePoint returns the code unit', () {
+      expect(scanner.peekCodePoint(), equals(codePoint));
+      expect(scanner.position, equals(5));
+    });
+
+    test('scanChar with the high surrogate moves into the pair', () {
+      expect(scanner.scanChar(highSurrogate), isTrue);
+      expect(scanner.position, equals(6));
+    });
+
+    test('scanChar with the code point moves past the pair', () {
+      expect(scanner.scanChar(codePoint), isTrue);
+      expect(scanner.position, equals(7));
+    });
+
+    test('expectChar with the high surrogate moves into the pair', () {
+      scanner.expectChar(highSurrogate);
+      expect(scanner.position, equals(6));
+    });
+
+    test('expectChar with the code point moves past the pair', () {
+      scanner.expectChar(codePoint);
+      expect(scanner.position, equals(7));
+    });
+  });
+
+  group('before an invalid surrogate pair', () {
+    // This surrogate pair is invalid because U+E000 is just outside the range
+    // of low surrogates. If it were interpreted as a surrogate pair anyway, the
+    // value would be U+110000, which is outside of the Unicode gamut.
+    const codePoint = 0x110000;
+    const highSurrogate = 0xD800;
+
+    late StringScanner scanner;
+    setUp(() {
+      scanner = StringScanner('foo: \uD800\uE000');
+      expect(scanner.scan('foo: '), isTrue);
+    });
+
+    test('readChar returns the high surrogate and moves into the pair', () {
+      expect(scanner.readChar(), equals(highSurrogate));
+      expect(scanner.position, equals(6));
+    });
+
+    test('readCodePoint returns the high surrogate and moves past the pair',
+        () {
+      expect(scanner.readCodePoint(), equals(highSurrogate));
+      expect(scanner.position, equals(6));
+    });
+
+    test('peekChar returns the high surrogate', () {
+      expect(scanner.peekChar(), equals(highSurrogate));
+      expect(scanner.position, equals(5));
+    });
+
+    test('peekCodePoint returns the high surrogate', () {
+      expect(scanner.peekCodePoint(), equals(highSurrogate));
+      expect(scanner.position, equals(5));
+    });
+
+    test('scanChar with the high surrogate moves into the pair', () {
+      expect(scanner.scanChar(highSurrogate), isTrue);
+      expect(scanner.position, equals(6));
+    });
+
+    test('scanChar with the fake code point returns false', () {
+      expect(scanner.scanChar(codePoint), isFalse);
+      expect(scanner.position, equals(5));
+    });
+
+    test('expectChar with the high surrogate moves into the pair', () {
+      scanner.expectChar(highSurrogate);
+      expect(scanner.position, equals(6));
+    });
+
+    test('expectChar with the fake code point fails', () {
+      expect(() => scanner.expectChar(codePoint), throwsRangeError);
+    });
+  });
+
+  group('a scanner constructed with a custom position', () {
+    test('starts scanning from that position', () {
+      final scanner = StringScanner('foo bar', position: 1);
+      expect(scanner.position, equals(1));
+      expect(scanner.rest, equals('oo bar'));
+
+      expect(scanner.scan(RegExp('oo.')), isTrue);
+      expect(scanner.lastMatch![0], equals('oo '));
+      expect(scanner.position, equals(4));
+      expect(scanner.rest, equals('bar'));
+    });
+
+    test('throws an ArgumentError if the position is -1', () {
+      expect(() => StringScanner('foo bar', position: -1), throwsArgumentError);
+    });
+
+    test('throws an ArgumentError if the position is beyond the string', () {
+      expect(() => StringScanner('foo bar', position: 8), throwsArgumentError);
+    });
+  });
+}
diff --git a/pkgs/string_scanner/test/utils.dart b/pkgs/string_scanner/test/utils.dart
new file mode 100644
index 0000000..ca03c06
--- /dev/null
+++ b/pkgs/string_scanner/test/utils.dart
@@ -0,0 +1,12 @@
+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// 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 'package:string_scanner/string_scanner.dart';
+import 'package:test/test.dart';
+
+/// Returns a matcher that asserts that a closure throws a
+/// [StringScannerException] with the given [text].
+Matcher throwsStringScannerException(String text) =>
+    throwsA(const TypeMatcher<StringScannerException>()
+        .having((e) => e.span!.text, 'span.text', text));