EditableText Cursor can be set to not blink for testing (flutter#20004)

* no blinking cursor

* debugDeterministicCursor: private -> public

* added documentation

* whitespace

Author: sandrasandeep

packages/flutter/lib/src/widgets/editable_text.dart

@@ -399,6 +399,12 @@ class EditableText extends StatefulWidget {
   /// Defaults to EdgeInserts.all(20.0).
   final EdgeInsets scrollPadding;
 
+  /// Setting this property to true makes the cursor stop blinking and stay visible on the screen continually.
+  /// This property is most useful for testing purposes.
+  ///
+  /// Defaults to false, resulting in a typical blinking cursor.
+  static bool debugDeterministicCursor = false;
+
   @override
   EditableTextState createState() => new EditableTextState();
 
@@ -881,7 +887,7 @@ class EditableTextState extends State<EditableText> with AutomaticKeepAliveClien
               textSpan: buildTextSpan(),
               value: _value,
               cursorColor: widget.cursorColor,
-              showCursor: _showCursor,
+              showCursor: EditableText.debugDeterministicCursor ? ValueNotifier<bool>(true) : _showCursor,
               hasFocus: _hasFocus,
               maxLines: widget.maxLines,
               selectionColor: widget.selectionColor,
@@ -1030,4 +1036,4 @@ class _Editable extends LeafRenderObjectWidget {
       ..cursorWidth = cursorWidth
       ..cursorRadius = cursorRadius;
   }
-}
+}