No multiline password fields (#42470)

Use assertions to prevent multiline password fields from being created

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

@@ -274,6 +274,7 @@ class CupertinoTextField extends StatefulWidget {
          !expands || (maxLines == null && minLines == null),
          'minLines and maxLines must be null when expands is true.',
        ),
+       assert(!obscureText || maxLines == 1, 'Obscured fields cannot be multiline.'),
        assert(maxLength == null || maxLength > 0),
        assert(clearButtonMode != null),
        assert(prefixMode != null),

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

@@ -344,6 +344,7 @@ class TextField extends StatefulWidget {
          !expands || (maxLines == null && minLines == null),
          'minLines and maxLines must be null when expands is true.',
        ),
+       assert(!obscureText || maxLines == 1, 'Obscured fields cannot be multiline.'),
        assert(maxLength == null || maxLength == TextField.noMaxLength || maxLength > 0),
        keyboardType = keyboardType ?? (maxLines == 1 ? TextInputType.text : TextInputType.multiline),
        toolbarOptions = toolbarOptions ?? (obscureText ?

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

@@ -139,6 +139,7 @@ class TextFormField extends FormField<String> {
          !expands || (maxLines == null && minLines == null),
          'minLines and maxLines must be null when expands is true.',
        ),
+       assert(!obscureText || maxLines == 1, 'Obscured fields cannot be multiline.'),
        assert(maxLength == null || maxLength > 0),
        assert(enableInteractiveSelection != null),
        super(

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

@@ -422,6 +422,7 @@ class EditableText extends StatefulWidget {
          !expands || (maxLines == null && minLines == null),
          'minLines and maxLines must be null when expands is true.',
        ),
+       assert(!obscureText || maxLines == 1, 'Obscured fields cannot be multiline.'),
        assert(autofocus != null),
        assert(rendererIgnoresPointer != null),
        assert(scrollPadding != null),

packages/flutter/test/material/text_field_test.dart

@@ -6530,7 +6530,7 @@ void main() {
       textAlign: TextAlign.end,
       textDirection: TextDirection.ltr,
       autofocus: true,
-      obscureText: true,
+      obscureText: false,
       autocorrect: false,
       maxLines: 10,
       maxLength: 100,
@@ -6554,7 +6554,6 @@ void main() {
       'decoration: InputDecoration(labelText: "foo")',
       'style: TextStyle(inherit: true, color: Color(0xff00ff00))',
       'autofocus: true',
-      'obscureText: true',
       'autocorrect: false',
       'maxLines: 10',
       'maxLength: 100',

packages/flutter/test/widgets/editable_text_test.dart

@@ -3350,6 +3350,38 @@ void main() {
     expect(scrollable.controller.position.pixels, equals(14));
     expect(scrollable.controller.position.pixels, equals(renderEditable.maxScrollExtent));
   }, skip: isBrowser);
+
+  testWidgets('obscured multiline fields throw an exception', (WidgetTester tester) async {
+    final TextEditingController controller = TextEditingController();
+    expect(
+      () {
+        EditableText(
+          backgroundCursorColor: cursorColor,
+          controller: controller,
+          cursorColor: cursorColor,
+          focusNode: focusNode,
+          maxLines: 1,
+          obscureText: true,
+          style: textStyle,
+        );
+      },
+      returnsNormally,
+    );
+    expect(
+      () {
+        EditableText(
+          backgroundCursorColor: cursorColor,
+          controller: controller,
+          cursorColor: cursorColor,
+          focusNode: focusNode,
+          maxLines: 2,
+          obscureText: true,
+          style: textStyle,
+        );
+      },
+      throwsAssertionError,
+    );
+  });
 }
 
 class MockTextSelectionControls extends Mock implements TextSelectionControls {