[web] Fix crash on old versions of Chrome (Intl.Segmenter missing) (flutter/engine#41986)

`Chrome < v87` have the `v8BreakIterator` API but don't have `Intl.Segmenter`. This caused crashes for some apps (see b/281377168).

This PR enhances our feature detection to take into account this API mismatch.

engine/src/flutter/lib/web_ui/lib/src/engine/browser_detection.dart

@@ -268,4 +268,5 @@ int _detectWebGLVersion() {
 }
 
 /// Whether the current browser supports the Chromium variant of CanvasKit.
-bool get browserSupportsCanvaskitChromium => domIntl.v8BreakIterator != null;
+bool get browserSupportsCanvaskitChromium =>
+    domIntl.v8BreakIterator != null && domIntl.Segmenter != null;

engine/src/flutter/lib/web_ui/test/canvaskit/canvaskit_api_test.dart

@@ -1826,17 +1826,22 @@ void _paragraphTests() {
   }, skip: isFirefox); // Intended: Headless firefox has no webgl support https://github.com/flutter/flutter/issues/109265
 
   group('getCanvasKitJsFileNames', () {
-    late dynamic oldV8BreakIterator = v8BreakIterator;
+    dynamic oldV8BreakIterator = v8BreakIterator;
+    dynamic oldIntlSegmenter = intlSegmenter;
+
     setUp(() {
       oldV8BreakIterator = v8BreakIterator;
+      oldIntlSegmenter = intlSegmenter;
     });
     tearDown(() {
       v8BreakIterator = oldV8BreakIterator;
+      intlSegmenter = oldIntlSegmenter;
       debugResetBrowserSupportsImageDecoder();
     });
 
     test('in Chromium-based browsers', () {
       v8BreakIterator = Object(); // Any non-null value.
+      intlSegmenter = Object(); // Any non-null value.
       browserSupportsImageDecoder = true;
 
       expect(getCanvasKitJsFileNames(CanvasKitVariant.full), <String>['canvaskit.js']);
@@ -1847,7 +1852,19 @@ void _paragraphTests() {
       ]);
     });
 
+    test('in older versions of Chromium-based browsers', () {
+      v8BreakIterator = Object(); // Any non-null value.
+      intlSegmenter = null; // Older versions of Chromium didn't have the Intl.Segmenter API.
+      browserSupportsImageDecoder = true;
+
+      expect(getCanvasKitJsFileNames(CanvasKitVariant.full), <String>['canvaskit.js']);
+      expect(getCanvasKitJsFileNames(CanvasKitVariant.chromium), <String>['chromium/canvaskit.js']);
+      expect(getCanvasKitJsFileNames(CanvasKitVariant.auto), <String>['canvaskit.js']);
+    });
+
     test('in other browsers', () {
+      intlSegmenter = Object(); // Any non-null value.
+
       v8BreakIterator = null;
       browserSupportsImageDecoder = true;
       expect(getCanvasKitJsFileNames(CanvasKitVariant.full), <String>['canvaskit.js']);
@@ -1901,3 +1918,9 @@ external dynamic get v8BreakIterator;
 
 @JS('window.Intl.v8BreakIterator')
 external set v8BreakIterator(dynamic x);
+
+@JS('window.Intl.Segmenter')
+external dynamic get intlSegmenter;
+
+@JS('window.Intl.Segmenter')
+external set intlSegmenter(dynamic x);

engine/src/flutter/lib/web_ui/test/engine/browser_detect_test.dart

@@ -157,17 +157,22 @@ void testMain() {
   });
 
   group('browserSupportsCanvasKitChromium', () {
-    late dynamic oldV8BreakIterator = v8BreakIterator;
+    dynamic oldV8BreakIterator = v8BreakIterator;
+    dynamic oldIntlSegmenter = intlSegmenter;
+
     setUp(() {
       oldV8BreakIterator = v8BreakIterator;
+      oldIntlSegmenter = intlSegmenter;
     });
     tearDown(() {
       v8BreakIterator = oldV8BreakIterator;
+      intlSegmenter = oldIntlSegmenter;
       debugResetBrowserSupportsImageDecoder();
     });
 
     test('Detect browsers that support CanvasKit Chromium', () {
       v8BreakIterator = Object(); // Any non-null value.
+      intlSegmenter = Object(); // Any non-null value.
       browserSupportsImageDecoder = true;
 
       expect(browserSupportsCanvaskitChromium, isTrue);
@@ -175,6 +180,7 @@ void testMain() {
 
     test('Detect browsers that do not support image codecs', () {
       v8BreakIterator = Object(); // Any non-null value.
+      intlSegmenter = Object(); // Any non-null value.
       browserSupportsImageDecoder = false;
 
       // TODO(mdebbar): we don't check image codecs for now.
@@ -184,6 +190,7 @@ void testMain() {
 
     test('Detect browsers that do not support v8BreakIterator', () {
       v8BreakIterator = null;
+      intlSegmenter = Object(); // Any non-null value.
       browserSupportsImageDecoder = true;
 
       expect(browserSupportsCanvaskitChromium, isFalse);
@@ -191,10 +198,18 @@ void testMain() {
 
     test('Detect browsers that support neither', () {
       v8BreakIterator = null;
+      intlSegmenter = Object(); // Any non-null value.
       browserSupportsImageDecoder = false;
 
       expect(browserSupportsCanvaskitChromium, isFalse);
     });
+
+    test('Detect browsers that support v8BreakIterator but no Intl.Segmenter', () {
+      v8BreakIterator = Object(); // Any non-null value.
+      intlSegmenter = null;
+
+      expect(browserSupportsCanvaskitChromium, isFalse);
+    });
   });
 
   group('OffscreenCanvas', () {
@@ -211,3 +226,9 @@ external dynamic get v8BreakIterator;
 
 @JS('window.Intl.v8BreakIterator')
 external set v8BreakIterator(dynamic x);
+
+@JS('window.Intl.Segmenter')
+external dynamic get intlSegmenter;
+
+@JS('window.Intl.Segmenter')
+external set intlSegmenter(dynamic x);