Add a sentinel value for TextStyle.height (#149049)

Fixes: flutter/flutter#58765

The rationale for the choice of the sentinel value: flutter/engine#52940
The exact value of `kTextHeightNone` should be kept as an implementation detail. It's unfortunate that the current value `0` is dangerously close to `TextStyle.height`'s valid domain. If we ever allow `TextStyle.height == 0` (which totally makes sense) then it shouldn't be difficult to change the const.

packages/flutter/lib/painting.dart

@@ -17,7 +17,7 @@
 ///    painting boxes.
 library painting;
 
-export 'dart:ui' show PlaceholderAlignment, Shadow, TextHeightBehavior, TextLeadingDistribution;
+export 'dart:ui' show PlaceholderAlignment, Shadow, TextHeightBehavior, TextLeadingDistribution, kTextHeightNone;
 
 export 'src/painting/alignment.dart';
 export 'src/painting/basic_types.dart';

packages/flutter/lib/src/painting/text_style.dart

@@ -8,6 +8,7 @@ import 'dart:ui' as ui show
   Shadow,
   StrutStyle,
   TextStyle,
+  kTextHeightNone,
   lerpDouble;
 
 import 'package:flutter/foundation.dart';
@@ -640,14 +641,13 @@ class TextStyle with Diagnosticable {
 
   /// The height of this text span, as a multiple of the font size.
   ///
-  /// When [height] is null or omitted, the line height will be determined
-  /// by the font's metrics directly, which may differ from the fontSize.
-  /// When [height] is non-null, the line height of the span of text will be a
-  /// multiple of [fontSize] and be exactly `fontSize * height` logical pixels
-  /// tall.
+  /// When [height] is [kTextHeightNone], the line height will be determined by
+  /// the font's metrics directly, which may differ from the fontSize. Otherwise
+  /// the line height of the span of text will be a multiple of [fontSize],
+  /// and be exactly `fontSize * height` logical pixels tall.
   ///
-  /// For most fonts, setting [height] to 1.0 is not the same as omitting or
-  /// setting height to null because the [fontSize] sets the height of the EM-square,
+  /// For most fonts, setting [height] to 1.0 is not the same as setting height
+  /// to [kTextHeightNone] because the [fontSize] sets the height of the EM-square,
   /// which is different than the font provided metrics for line height. The
   /// following diagram illustrates the difference between the font-metrics
   /// defined line height and the line height produced with `height: 1.0`
@@ -954,7 +954,8 @@ class TextStyle with Diagnosticable {
   /// [TextStyle] with a [FontWeight.w300].
   ///
   /// If the underlying values are null, then the corresponding factors and/or
-  /// deltas must not be specified.
+  /// deltas must not be specified. Additionally, if [height] is [kTextHeightNone]
+  /// it will not be modified by this method.
   ///
   /// If [foreground] is specified on this object, then applying [color] here
   /// will have no effect and if [background] is specified on this object, then
@@ -1014,7 +1015,7 @@ class TextStyle with Diagnosticable {
       letterSpacing: letterSpacing == null ? null : letterSpacing! * letterSpacingFactor + letterSpacingDelta,
       wordSpacing: wordSpacing == null ? null : wordSpacing! * wordSpacingFactor + wordSpacingDelta,
       textBaseline: textBaseline ?? this.textBaseline,
-      height: height == null ? null : height! * heightFactor + heightDelta,
+      height: (height == null || height == ui.kTextHeightNone) ? height : height! * heightFactor + heightDelta,
       leadingDistribution: leadingDistribution ?? this.leadingDistribution,
       locale: locale ?? this.locale,
       foreground: foreground,

packages/flutter/test/painting/text_painter_test.dart

@@ -1696,6 +1696,17 @@ void main() {
     );
   });
 
+  test('kTextHeightNone unsets the text height multiplier', () {
+    final TextPainter painter = TextPainter(
+      textDirection: TextDirection.ltr,
+      text: const TextSpan(
+        style: TextStyle(fontSize: 10, height: 1000),
+        children: <TextSpan>[TextSpan(text: 'A', style: TextStyle(height: kTextHeightNone))],
+      ),
+    )..layout();
+    expect(painter.height, 10);
+  });
+
   test('TextPainter dispatches memory events', () async {
     await expectLater(
       await memoryEvents(() => TextPainter().dispose(), TextPainter),

packages/flutter/test/painting/text_style_test.dart

@@ -570,6 +570,11 @@ void main() {
       style.apply(leadingDistribution: TextLeadingDistribution.proportional).leadingDistribution,
       TextLeadingDistribution.proportional,
     );
+
+    expect(
+      const TextStyle(height: kTextHeightNone).apply(heightFactor: 1000, heightDelta: 1000).height,
+      kTextHeightNone,
+    );
   });
 
   test('TextStyle fontFamily and package', () {