Updated the gallery text fields demo (#15362)

examples/flutter_gallery/test/demo/material/text_form_field_demo_test.dart

@@ -16,21 +16,36 @@ void main() {
     final Finder nameField = find.widgetWithText(TextFormField, 'Name *');
     expect(nameField, findsOneWidget);
 
+    final Finder passwordField = find.widgetWithText(TextFormField, 'Password *');
+    expect(passwordField, findsOneWidget);
+
     await tester.enterText(nameField, '');
+    // The submit button isn't initially visible. Drag it into view so that
+    // it will see the tap.
+    await tester.drag(nameField, const Offset(0.0, -1200.0));
+    await tester.pumpAndSettle();
     await tester.tap(submitButton);
-    await tester.pump();
+    await tester.pumpAndSettle();
+
+    // Now drag the password field (the submit button will be obscured by
+    // the snackbar) and expose the name field again.
+    await tester.drag(passwordField, const Offset(0.0, 1200.0));
+    await tester.pumpAndSettle();
     expect(find.text('Name is required.'), findsOneWidget);
     expect(find.text('Please enter only alphabetical characters.'), findsNothing);
-
     await tester.enterText(nameField, '#');
+
+    // Make the submit button visible again (by dragging the name field), so
+    // it will see the tap.
+    await tester.drag(nameField, const Offset(0.0, -1200.0));
     await tester.tap(submitButton);
-    await tester.pump();
+    await tester.pumpAndSettle();
     expect(find.text('Name is required.'), findsNothing);
     expect(find.text('Please enter only alphabetical characters.'), findsOneWidget);
 
     await tester.enterText(nameField, 'Jane Doe');
     await tester.tap(submitButton);
-    await tester.pump();
+    await tester.pumpAndSettle();
     expect(find.text('Name is required.'), findsNothing);
     expect(find.text('Please enter only alphabetical characters.'), findsNothing);
   });

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

@@ -60,7 +60,9 @@ class TextFormField extends FormField<String> {
     bool autofocus: false,
     bool obscureText: false,
     bool autocorrect: true,
+    bool maxLengthEnforced: true,
     int maxLines: 1,
+    int maxLength,
     ValueChanged<String> onFieldSubmitted,
     FormFieldSetter<String> onSaved,
     FormFieldValidator<String> validator,
@@ -71,7 +73,9 @@ class TextFormField extends FormField<String> {
        assert(autofocus != null),
        assert(obscureText != null),
        assert(autocorrect != null),
+       assert(maxLengthEnforced != null),
        assert(maxLines == null || maxLines > 0),
+       assert(maxLength == null || maxLength > 0),
        super(
     key: key,
     initialValue: controller != null ? controller.text : (initialValue ?? ''),
@@ -91,7 +95,9 @@ class TextFormField extends FormField<String> {
         autofocus: autofocus,
         obscureText: obscureText,
         autocorrect: autocorrect,
+        maxLengthEnforced: maxLengthEnforced,
         maxLines: maxLines,
+        maxLength: maxLength,
         onChanged: field.onChanged,
         onSubmitted: onFieldSubmitted,
         inputFormatters: inputFormatters,