Handle Back Delete.

Runtime/rendering/editable.cs

         void _handleDelete() {
             var selection = this.selection;
             if (selection.textAfter(this.text.text).isNotEmpty()) {
-                this.textSelectionDelegate.textEditingValue = new TextEditingValue(
-                    text: selection.textBefore(this.text.text)
-                          + selection.textAfter(this.text.text).Substring(1),
-                    selection: TextSelection.collapsed(offset: selection.start)
-                );
+                if (char.IsLowSurrogate(this.text.text[selection.end])) {
+                    this.textSelectionDelegate.textEditingValue = new TextEditingValue(
+                        text: selection.textBefore(this.text.text)
+                              + selection.textAfter(this.text.text).Substring(2),
+                        selection: TextSelection.collapsed(offset: selection.start)
+                    );
+                }
+                else {
+                    this.textSelectionDelegate.textEditingValue = new TextEditingValue(
+                        text: selection.textBefore(this.text.text)
+                              + selection.textAfter(this.text.text).Substring(1),
+                        selection: TextSelection.collapsed(offset: selection.start)
+                    );
+                }
             }
             else {
                 this.textSelectionDelegate.textEditingValue = new TextEditingValue(