bug fix when performing selection on multi-line paragraph

Author: zhuxingwei

Runtime/rendering/paragraph.cs

@@ -266,7 +266,7 @@ void _handleKeyEvent(RawKeyEvent keyEvent) {
 
             if (kcmd == KeyCommand.Copy) {
                 Clipboard.setData(
-                    new ClipboardData(text: this.selection.textInside(this.text.text))
+                    new ClipboardData(text: this.selection.textInside(this.text.toPlainText()))
                 );
             }
         }
@@ -284,7 +284,6 @@ public override void detach() {
         public void selectPositionAt(Offset from = null, Offset to = null, SelectionChangedCause? cause = null) {
             D.assert(cause != null);
             D.assert(from != null);
-            this._layoutText(this.constraints.maxWidth);
             if (true) {
                 TextPosition fromPosition =
                     this._textPainter.getPositionForOffset(this.globalToLocal(from));

Runtime/widgets/selectable_text.cs

@@ -36,7 +36,7 @@ public SelectableText(string data,
             this.selectionColor = selectionColor;
         }
 
-        SelectableText(TextSpan textSpan,
+        public SelectableText(TextSpan textSpan,
             Key key = null,
             TextStyle style = null,
             TextAlign? textAlign = null,