[web] Implement CanvasParagraph.getLineBoundary

lib/web_ui/lib/src/engine/text/canvas_paragraph.dart

@@ -242,9 +242,19 @@ class CanvasParagraph implements EngineParagraph {
 
   @override
   ui.TextRange getLineBoundary(ui.TextPosition position) {
-    // TODO(mdebbar): After layout, line metrics should be available and can be
-    // used to determine the line boundary of the given `position`.
-    return ui.TextRange.empty;
+    final int index = position.offset;
+    final List<EngineLineMetrics> lines = computeLineMetrics();
+
+    int i;
+    for (i = 0; i < lines.length - 1; i++) {
+      final EngineLineMetrics line = lines[i];
+      if (index >= line.startIndex && index < line.endIndex) {
+        break;
+      }
+    }
+
+    final EngineLineMetrics line = lines[i];
+    return ui.TextRange(start: line.startIndex, end: line.endIndex);
   }
 
   @override

lib/web_ui/test/text/canvas_paragraph_test.dart

@@ -547,6 +547,60 @@ void testMain() async {
       );
     });
   });
+
+  group('$CanvasParagraph.getLineBoundary', () {
+    test('single-line', () {
+      final CanvasParagraph paragraph = rich(ahemStyle, (builder) {
+        builder.addText('One single line');
+      })
+        ..layout(constrain(400.0));
+
+      // "One single line".length == 15
+      for (int i = 0; i < 15; i++) {
+        expect(
+          paragraph.getLineBoundary(ui.TextPosition(offset: i)),
+          ui.TextRange(start: 0, end: 15),
+          reason: 'failed at offset $i',
+        );
+      }
+    });
+
+    test('multi-line', () {
+      final CanvasParagraph paragraph = rich(ahemStyle, (builder) {
+        builder.addText('First line\n');
+        builder.addText('Second line\n');
+        builder.addText('Third line');
+      })
+        ..layout(constrain(400.0));
+
+      // "First line\n".length == 11
+      for (int i = 0; i < 11; i++) {
+        expect(
+          paragraph.getLineBoundary(ui.TextPosition(offset: i)),
+          ui.TextRange(start: 0, end: 11),
+          reason: 'failed at offset $i',
+        );
+      }
+
+      // "Second line\n".length == 12
+      for (int i = 11; i < 23; i++) {
+        expect(
+          paragraph.getLineBoundary(ui.TextPosition(offset: i)),
+          ui.TextRange(start: 11, end: 23),
+          reason: 'failed at offset $i',
+        );
+      }
+
+      // "Third line".length == 10
+      for (int i = 23; i < 33; i++) {
+        expect(
+          paragraph.getLineBoundary(ui.TextPosition(offset: i)),
+          ui.TextRange(start: 23, end: 33),
+          reason: 'failed at offset $i',
+        );
+      }
+    });
+  });
 }
 
 /// Shortcut to create a [ui.TextBox] with an optional [ui.TextDirection].