Keep the selection within bounds in deleteSurroundingText

If the text is empty, the IME may call deleteSurroundingText(1, 0) if the user
hits the delete key.  The selection should remain at position 0 if this
happens.

packages/flutter/lib/src/material/input.dart

@@ -37,10 +37,10 @@ class Input extends Scrollable {
   final StringValueChanged onChanged;
   final StringValueSubmitted onSubmitted;
 
-  _InputState createState() => new _InputState();
+  InputState createState() => new InputState();
 }
 
-class _InputState extends ScrollableState<Input> {
+class InputState extends ScrollableState<Input> {
   String _value;
   EditableString _editableValue;
   KeyboardHandle _keyboardHandle = KeyboardHandle.unattached;
@@ -48,6 +48,8 @@ class _InputState extends ScrollableState<Input> {
   double _contentWidth = 0.0;
   double _containerWidth = 0.0;
 
+  EditableString get editableValue => _editableValue;
+
   void initState() {
     super.initState();
     _value = config.initialValue;

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

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:math' as math;
 
 import 'package:mojo_services/keyboard/keyboard.mojom.dart';
 import 'package:flutter/painting.dart';
@@ -112,8 +113,9 @@ class EditableString implements KeyboardClient {
     _delete(afterRange);
     _delete(beforeRange);
     selection = new TextRange(
-        start: selection.start - beforeLength,
-        end: selection.end - beforeLength);
+      start: math.max(selection.start - beforeLength, 0),
+      end: math.max(selection.end - beforeLength, 0)
+    );
     onUpdated();
   }
 

packages/unit/test/widget/input_test.dart

@@ -101,4 +101,38 @@ void main() {
       checkCursorToggle();
     });
   });
+
+  test('Selection remains valid', () {
+    testWidgets((WidgetTester tester) {
+      GlobalKey inputKey = new GlobalKey();
+      String inputValue;
+
+      Widget builder() {
+        return new Center(
+          child: new Input(
+            key: inputKey,
+            placeholder: 'Placeholder'
+          )
+        );
+      }
+
+      tester.pumpWidget(builder());
+
+      const String testValue = 'ABC';
+      mockKeyboard.client.commitText(testValue, testValue.length);
+      InputState input = tester.findStateOfType(InputState);
+
+      // Delete characters and verify that the selection follows the length
+      // of the text.
+      for (int i = 0; i < testValue.length; i++) {
+        mockKeyboard.client.deleteSurroundingText(1, 0);
+        expect(input.editableValue.selection.start, equals(testValue.length - i - 1));
+      }
+
+      // Delete a characters when the text is empty.  The selection should
+      // remain at zero.
+      mockKeyboard.client.deleteSurroundingText(1, 0);
+      expect(input.editableValue.selection.start, equals(0));
+    });
+  });
 }