Allow disabling formatting for a region of code. (#1522)

Allow disabling formatting for a region of code.

There are two special line comments:

```
// dart format off
// dart format on
```

Any code between those keeps its original formatting completely,
including indentation. It just splats the original unformatted code into
the output.

Author: munificent

CHANGELOG.md

@@ -3,6 +3,10 @@
 * Allow passing a language version to `DartFomatter()`. Formatted code will be
   parsed at that version. If omitted, defaults to the latest version. In a
   future release, this parameter will become required.
+* Allow opting out of formatting for a region of code using `// dart format off`
+  and `// dart format on` comments. Note: This only works using the new tall
+  style and requires passing the `--enable-experiment=tall-style` experiment
+  flag (#361).
 * Preserve type parameters on old-style function-typed formals that also use
   `this.` or `super.` (#1321).
 * Remove temporary work around for analyzer 6.2.0 from dart_style 2.3.6.

benchmark/run.dart

@@ -160,7 +160,7 @@ double _runTrial(DartFormatter formatter, ParseStringResult parseResult,
       result = visitor.run(parseResult.unit).text;
     } else {
       var visitor = AstNodeVisitor(formatter, parseResult.lineInfo, source);
-      result = visitor.run(parseResult.unit).text;
+      result = visitor.run(source, parseResult.unit).text;
     }
   }
 

example/format.dart

@@ -25,7 +25,7 @@ void main(List<String> args) {
   class C {}
   ''');
 
-  _runTest('selection/selection.stmt', 2);
+  _runTest('other/selection.stmt', 2);
 }
 
 void _formatStmt(String source, {bool tall = true, int pageWidth = 40}) {
@@ -68,7 +68,7 @@ void _drawRuler(String label, int width) {
 /// directory.
 Future<void> _runTest(String path, int line,
     {int pageWidth = 40, bool tall = true}) async {
-  var testFile = await TestFile.read(path);
+  var testFile = await TestFile.read('${tall ? 'tall' : 'short'}/$path');
   var formatTest = testFile.tests.firstWhere((test) => test.line == line);
 
   var formatter = DartFormatter(