Apply class modifiers to all the classes. (#1575)

Now that we've bumped the major version and can make breaking changes,
I went ahead and locked down all of the classes. It mostly matters for
the public ones, but I went ahead and applied it to the internal ones
too just to see if it makes things easier to maintain.

Author: munificent

CHANGELOG.md

@@ -67,6 +67,12 @@
   If `--stdin-name` and `--language-version` are both omitted, then parses
   stdin using the latest supported language version.
 
+* **Apply class modifiers to API classes.** The dart_style package exposes only
+  a few classes in its public API: `DartFormatter`, `SourceCode`,
+  `FormatterException`, and `UnexpectedOutputException`. None were ever
+  intended to be extended or implemented. They are now all marked `final` to
+  make that intention explicit.
+
 ## 2.3.7
 
 * Allow passing a language version to `DartFomatter()`. Formatted code will be

lib/src/back_end/code.dart

@@ -177,7 +177,7 @@ enum _Marker { start, end }
 
 /// Traverses a [Code] tree and produces the final string of output code and
 /// the selection markers, if any.
-class _StringBuilder {
+final class _StringBuilder {
   /// Pre-calculated whitespace strings for various common levels of
   /// indentation.
   ///

lib/src/back_end/code_writer.dart

@@ -20,7 +20,7 @@ import 'solution_cache.dart';
 /// an instance of this class. It has methods that the piece can call to add
 /// output text to the resulting code, recursively format child pieces, insert
 /// whitespace, etc.
-class CodeWriter {
+final class CodeWriter {
   final int _pageWidth;
 
   /// Previously cached formatted subtrees.
@@ -415,7 +415,7 @@ enum Whitespace {
 }
 
 /// A level of indentation in the indentation stack.
-class _Indent {
+final class _Indent {
   /// The total number of spaces of indentation.
   final int indent;
 

lib/src/back_end/solution.dart

@@ -17,7 +17,7 @@ import 'solution_cache.dart';
 /// of the pieces in the tree so they can format themselves. That in turn
 /// yields a total number of overflow characters, cost, and formatted output,
 /// which are all stored here.
-class Solution implements Comparable<Solution> {
+final class Solution implements Comparable<Solution> {
   /// The states that pieces have been bound to.
   ///
   /// Note that order that keys are inserted into this map is significant. When

lib/src/back_end/solution_cache.dart

@@ -22,7 +22,7 @@ import 'solver.dart';
 /// the same child Piece and wanting to format it separately with the same
 /// indentation. When that happens, sharing this cache allows us to reuse that
 /// cached subtree Solution.
-class SolutionCache {
+final class SolutionCache {
   final _cache = <_Key, Solution>{};
 
   /// Returns a previously cached solution for formatting [root] with leading

lib/src/back_end/solver.dart

@@ -42,7 +42,7 @@ const _maxAttempts = 10000;
 ///     use it across different Solutions. This enables us to both divide and
 ///     conquer the Piece tree and solve portions separately, while also
 ///     reusing work across different solutions.
-class Solver {
+final class Solver {
   final SolutionCache _cache;
 
   final int _pageWidth;

lib/src/cli/format_command.dart

@@ -14,7 +14,7 @@ import 'output.dart';
 import 'show.dart';
 import 'summary.dart';
 
-class FormatCommand extends Command<int> {
+final class FormatCommand extends Command<int> {
   @override
   String get name => 'format';
 

lib/src/cli/formatter_options.dart

@@ -15,7 +15,7 @@ import 'summary.dart';
 const dartStyleVersion = '2.3.7';
 
 /// Global options that affect how the formatter produces and uses its outputs.
-class FormatterOptions {
+final class FormatterOptions {
   /// The language version formatted code should be parsed at or `null` if not
   /// specified.
   final Version? languageVersion;

lib/src/cli/summary.dart

@@ -8,7 +8,7 @@ import '../source_code.dart';
 import 'formatter_options.dart';
 
 /// The kind of summary shown after all formatting is complete.
-class Summary {
+final class Summary {
   static const Summary none = Summary._();
 
   /// Creates a Summary that tracks how many files were formatted and the total
@@ -41,7 +41,7 @@ class Summary {
 }
 
 /// Tracks how many files were formatted and the total time.
-class _LineSummary extends Summary {
+final class _LineSummary extends Summary {
   final DateTime _start = DateTime.now();
 
   /// The number of processed files.
@@ -81,7 +81,7 @@ class _LineSummary extends Summary {
 }
 
 /// Reports how long it took for format each file.
-class _ProfileSummary implements Summary {
+final class _ProfileSummary implements Summary {
   /// The files that have been started but have not completed yet.
   ///
   /// Maps a file label to the time that it started being formatted.

lib/src/constants.dart

@@ -13,7 +13,7 @@ const tallStyleExperimentFlag = 'tall-style';
 
 /// Constants for the cost heuristics used to determine which set of splits is
 /// most desirable.
-class Cost {
+final class Cost {
   /// The cost of splitting after the `=>` in a lambda or arrow-bodied member.
   ///
   /// We make this zero because there is already a span around the entire body
@@ -64,7 +64,7 @@ class Cost {
 }
 
 /// Constants for the number of spaces for various kinds of indentation.
-class Indent {
+final class Indent {
   /// Reset back to no indentation.
   static const none = 0;