Re-add deprecated method for plugin migration compatibility. (#34006)

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

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 
 import 'basic_types.dart';
 import 'text_painter.dart';
+import 'text_span.dart';
 import 'text_style.dart';
 
 /// Mutable wrapper of an integer that can be passed by reference to track a
@@ -103,6 +104,15 @@ abstract class InlineSpan extends DiagnosticableTree {
   /// [Paragraph] objects can be drawn on [Canvas] objects.
   void build(ui.ParagraphBuilder builder, { double textScaleFactor = 1.0, List<PlaceholderDimensions> dimensions });
 
+  // TODO(garyq): Remove this after next stable release.
+  /// Walks this [TextSpan] and any descendants in pre-order and calls `visitor`
+  /// for each span that has content.
+  ///
+  /// When `visitor` returns true, the walk will continue. When `visitor` returns
+  /// false, then the walk will end.
+  @Deprecated('Use to visitChildren instead')
+  bool visitTextSpan(bool visitor(TextSpan span));
+
   /// Walks this [InlineSpan] and any descendants in pre-order and calls `visitor`
   /// for each span that has content.
   ///

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

@@ -60,6 +60,15 @@ abstract class PlaceholderSpan extends InlineSpan {
     }
   }
 
+  // TODO(garyq): Remove this after next stable release.
+  /// The [visitTextSpan] method is invalid on [PlaceholderSpan]s
+  @override
+  @Deprecated('Use to visitChildren instead')
+  bool visitTextSpan(bool visitor(TextSpan span)) {
+    assert(false, 'visitTextSpan is deprecated. Use visitChildren to support InlineSpans');
+    return false;
+  }
+
   /// Populates the `semanticsOffsets` and `semanticsElements` with the appropriate data
   /// to be able to construct a [SemanticsNode].
   ///

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

@@ -164,8 +164,13 @@ class TextPainter {
   /// The (potentially styled) text to paint.
   ///
   /// After this is set, you must call [layout] before the next call to [paint].
-  ///
   /// This and [textDirection] must be non-null before you call [layout].
+  ///
+  /// The [InlineSpan] this provides is in the form of a tree that may contain
+  /// multiple instances of [TextSpan]s and [WidgetSpan]s. To obtain a plaintext
+  /// representation of the contents of this [TextPainter], use [InlineSpan.toPlainText]
+  /// to get the full contents of all nodes in the tree. [TextSpan.text] will
+  /// only provide the contents of the first node in the tree.
   InlineSpan get text => _text;
   InlineSpan _text;
   set text(InlineSpan value) {

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

@@ -205,8 +205,11 @@ class TextSpan extends InlineSpan {
       builder.pop();
   }
 
-  /// Walks this text span and its descendants in pre-order and calls [visitor]
+  /// Walks this [TextSpan] and its descendants in pre-order and calls [visitor]
   /// for each span that has text.
+  ///
+  /// When `visitor` returns true, the walk will continue. When `visitor` returns
+  /// false, then the walk will end.
   @override
   bool visitChildren(InlineSpanVisitor visitor) {
     if (text != null) {
@@ -222,6 +225,30 @@ class TextSpan extends InlineSpan {
     return true;
   }
 
+  // TODO(garyq): Remove this after next stable release.
+  /// Walks this [TextSpan] and any descendants in pre-order and calls `visitor`
+  /// for each span that has content.
+  ///
+  /// When `visitor` returns true, the walk will continue. When `visitor` returns
+  /// false, then the walk will end.
+  @override
+  @Deprecated('Use to visitChildren instead')
+  bool visitTextSpan(bool visitor(TextSpan span)) {
+    if (text != null) {
+      if (!visitor(this))
+        return false;
+    }
+    if (children != null) {
+      for (InlineSpan child in children) {
+        assert(child is TextSpan, 'visitTextSpan is deprecated. Use visitChildren to support InlineSpans');
+        final TextSpan textSpanChild = child;
+        if (!textSpanChild.visitTextSpan(visitor))
+          return false;
+      }
+    }
+    return true;
+  }
+
   /// Returns the text span that contains the given position in the text.
   @override
   InlineSpan getSpanForPositionVisitor(TextPosition position, Accumulator offset) {