Add default-no-clip tests to more buttons (#21012)

Note that such tests already exist for RaisedButton and MaterialButton

packages/flutter/test/material/buttons_test.dart

@@ -520,4 +520,22 @@ void main() {
 
     expect(tester.getSize(find.byKey(key2)), const Size(88.0, 36.0));
   });
+
+  testWidgets('RaisedButton has no clip by default', (WidgetTester tester) async{
+    await tester.pumpWidget(
+      new Directionality(
+          textDirection: TextDirection.ltr,
+          child: new Material(
+            child: new RaisedButton(
+              onPressed: () { /* to make sure the button is enabled */ },
+            ),
+          )
+      ),
+    );
+
+    expect(
+        tester.renderObject(find.byType(RaisedButton)),
+        paintsExactlyCountTimes(#clipPath, 0)
+    );
+  });
 }

packages/flutter/test/material/flat_button_test.dart

@@ -6,6 +6,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter/rendering.dart';
 
+import '../rendering/mock_canvas.dart';
+
 void main() {
   testWidgets('FlatButton implements debugFillDescription', (WidgetTester tester) async {
     final DiagnosticPropertiesBuilder builder = new DiagnosticPropertiesBuilder();
@@ -29,4 +31,23 @@ void main() {
       'splashColor: Color(0xff9e9e9e)',
     ]);
   });
+
+  testWidgets('FlatButton has no clip by default', (WidgetTester tester) async{
+    await tester.pumpWidget(
+      new Directionality(
+        textDirection: TextDirection.ltr,
+        child: new Material(
+          child: new FlatButton(
+            child: new Container(),
+            onPressed: () { /* to make sure the button is enabled */ },
+          ),
+        )
+      ),
+    );
+
+    expect(
+        tester.renderObject(find.byType(FlatButton)),
+        paintsExactlyCountTimes(#clipPath, 0)
+    );
+  });
 }

packages/flutter/test/material/floating_action_button_test.dart

@@ -9,6 +9,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../rendering/mock_canvas.dart';
 import '../widgets/semantics_tester.dart';
 
 void main() {
@@ -438,4 +439,22 @@ void main() {
       skip: !Platform.isLinux,
     );
   });
+
+  testWidgets('Floating Action Button has no clip by default', (WidgetTester tester) async{
+    await tester.pumpWidget(
+      new Directionality(
+          textDirection: TextDirection.ltr,
+          child: new Material(
+            child: new FloatingActionButton(
+              onPressed: () { /* to make sure the button is enabled */ },
+            ),
+          )
+      ),
+    );
+
+    expect(
+        tester.renderObject(find.byType(FloatingActionButton)),
+        paintsExactlyCountTimes(#clipPath, 0)
+    );
+  });
 }