Don't crash when TextField w/ explicit controller is activated via a11y (#17892)

Fixes #17801.

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

@@ -506,8 +506,8 @@ class _TextFieldState extends State<TextField> with AutomaticKeepAliveClientMixi
 
     return new Semantics(
       onTap: () {
-        if (!_controller.selection.isValid)
+        if (!_effectiveController.selection.isValid)
-          _controller.selection = new TextSelection.collapsed(offset: _controller.text.length);
+          _effectiveController.selection = new TextSelection.collapsed(offset: _effectiveController.text.length);
         _requestKeyboard();
       },
       child: new IgnorePointer(

packages/flutter/test/material/text_field_test.dart

@@ -2059,6 +2059,76 @@ void main() {
     semantics.dispose();
   });
 
+  testWidgets('Can activate TextField with explicit controller via semantics ', (WidgetTester tester) async {
+    // Regression test for https://github.com/flutter/flutter/issues/17801
+
+    const String textInTextField = 'Hello';
+
+    final SemanticsTester semantics = new SemanticsTester(tester);
+    final SemanticsOwner semanticsOwner = tester.binding.pipelineOwner.semanticsOwner;
+    final TextEditingController controller = new TextEditingController()
+      ..text = textInTextField;
+    final Key key = new UniqueKey();
+
+    await tester.pumpWidget(
+      overlay(
+        child: new TextField(
+          key: key,
+          controller: controller,
+        ),
+      ),
+    );
+
+    const int inputFieldId = 1;
+
+    expect(semantics, hasSemantics(
+      new TestSemantics.root(
+        children: <TestSemantics>[
+          new TestSemantics(
+            id: inputFieldId,
+            flags: <SemanticsFlag>[SemanticsFlag.isTextField],
+            actions: <SemanticsAction>[SemanticsAction.tap],
+            value: textInTextField,
+            textDirection: TextDirection.ltr,
+          ),
+        ],
+      ),
+      ignoreRect: true, ignoreTransform: true,
+    ));
+
+    semanticsOwner.performAction(inputFieldId, SemanticsAction.tap);
+    await tester.pump();
+
+    expect(semantics, hasSemantics(
+      new TestSemantics.root(
+        children: <TestSemantics>[
+          new TestSemantics(
+            id: inputFieldId,
+            flags: <SemanticsFlag>[
+              SemanticsFlag.isTextField,
+              SemanticsFlag.isFocused,
+            ],
+            actions: <SemanticsAction>[
+              SemanticsAction.tap,
+              SemanticsAction.moveCursorBackwardByCharacter,
+              SemanticsAction.setSelection,
+              SemanticsAction.paste,
+            ],
+            value: textInTextField,
+            textDirection: TextDirection.ltr,
+            textSelection: const TextSelection(
+              baseOffset: textInTextField.length,
+              extentOffset: textInTextField.length,
+            ),
+          ),
+        ],
+      ),
+      ignoreRect: true, ignoreTransform: true,
+    ));
+
+    semantics.dispose();
+  });
+
   testWidgets('TextField throws when not descended from a Material widget', (WidgetTester tester) async {
     const Widget textField = const TextField();
     await tester.pumpWidget(textField);