Use exact Ahem and exact dimensions for Cupertino button test (#8561)

* Use exact Ahem and exact dimensions for test

* use ‘px’ everywhere

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

@@ -24,7 +24,7 @@ final TextStyle _kBackgroundButtonTextStyle = _kButtonTextStyle.copyWith(
 
 const EdgeInsets _kButtonPadding = const EdgeInsets.all(16.0);
 const EdgeInsets _kBackgroundButtonPadding =
-const EdgeInsets.symmetric(vertical: 16.0, horizontal: 64.0);
+    const EdgeInsets.symmetric(vertical: 16.0, horizontal: 64.0);
 
 /// An iOS style button.
 ///

packages/flutter/test/cupertino/button_test.dart

@@ -7,56 +7,66 @@ import 'package:flutter/scheduler.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+const TextStyle testStyle = const TextStyle(
+  fontFamily: 'Ahem',
+  fontSize: 10.0,
+);
+
 void main() {
   testWidgets('Layout minimum size', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(child: new CupertinoButton(child: new Text(' '), onPressed: null))
+      new Center(child: new CupertinoButton(
+        child: new Text('X', style: testStyle),
+        onPressed: null,
+      ))
     );
     RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
     expect(
       buttonBox.size,
-      greaterThanOrEqualTo(const Size.square(48.0)),
-    );
-    expect(
-      buttonBox.size,
-      lessThan(const Size.square(100.0)),
+      // 1 10px character + 16px * 2 is smaller than the 48px minimum.
+      const Size.square(48.0),
     );
   });
 
   testWidgets('Size grows with text', (WidgetTester tester) async {
     await tester.pumpWidget(
-      new Center(child: new CupertinoButton(child: new Text('blah blah blah'), onPressed: null))
+      new Center(child: new CupertinoButton(
+        child: new Text('XXXX', style: testStyle),
+        onPressed: null,
+      ))
     );
     RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
     expect(
       buttonBox.size.width,
-      greaterThan(48.0),
+      // 4 10px character + 16px * 2 = 72.
+      72.0,
     );
   });
 
   testWidgets('Button with background is wider', (WidgetTester tester) async {
     await tester.pumpWidget(new Center(child: new CupertinoButton(
-      child: new Text(' '),
+      child: new Text('X', style: testStyle),
       onPressed: null,
       color: new Color(0xFFFFFFFF),
     )));
     RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
     expect(
       buttonBox.size.width,
-      greaterThan(120.0),
+      // 1 10px character + 64 * 2 = 138 for buttons with background.
+      138.0,
     );
   });
 
   testWidgets('Custom padding', (WidgetTester tester) async {
     await tester.pumpWidget(new Center(child: new CupertinoButton(
-      child: new Text(' '),
+      child: new Text(' ', style: testStyle),
       onPressed: null,
       padding: new EdgeInsets.all(100.0),
     )));
     RenderBox buttonBox = tester.renderObject(find.byType(CupertinoButton));
     expect(
       buttonBox.size,
-      greaterThan(const Size.square(100.0)),
+      const Size.square(210.0),
     );
   });