Add web API

Author: gspencergoog

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

@@ -291,6 +291,13 @@ class EngineParagraph implements ui.Paragraph {
     return <int>[start, end];
   }
 
+  @override
+  List<int> getLineBoundary(int offset) {
+    // TODO(flutter_web): https://github.com/flutter/flutter/issues/39537
+    // Depends upon LineMetrics measurement.
+    return null;
+  }
+
   @override
   List<ui.LineMetrics> computeLineMetrics() {
     // TODO(flutter_web): https://github.com/flutter/flutter/issues/39537

lib/web_ui/lib/src/ui/text.dart

@@ -1027,6 +1027,8 @@ class LineMetrics {
     this.left,
     this.baseline,
     this.lineNumber,
+    this.startIndex,
+    this.endIndex,
   });
 
   @pragma('vm:entry-point')
@@ -1040,6 +1042,8 @@ class LineMetrics {
     this.left,
     this.baseline,
     this.lineNumber,
+    this.startIndex,
+    this.endIndex,
   );
 
   final bool hardBreak;
@@ -1059,6 +1063,10 @@ class LineMetrics {
   final double baseline;
 
   final int lineNumber;
+
+  final int startIndex;
+
+  final int endIndex;
 }
 
 /// A paragraph of text.
@@ -1160,6 +1168,16 @@ abstract class Paragraph {
   /// http://www.unicode.org/reports/tr29/#Word_Boundaries
   List<int> getWordBoundary(int offset);
 
+  /// Returns the [start, end] of the line at the given offset.
+  ///
+  /// The newline (if any) is returned as part of the range.
+  ///
+  /// This can potentially be expensive, since it needs to compute the line
+  /// metrics, so use it sparingly. If higher performance is needed, caching the
+  /// results of [computeLineMetrics] is recommended (which also contains the
+  /// start and end of each line).
+  List<int> getLineBoundary(int offset);
+
   /// Returns a list of text boxes that enclose all placeholders in the paragraph.
   ///
   /// The order of the boxes are in the same order as passed in through [addPlaceholder].