Make dividers one device pixel thick as defined in Material design spec (#10523)

* Make dividers one device pixel thick as defined in Material design spec

* Updated divider test to check stroke width

* Clarified dividers with 0 height in the docs

* Updated Divider.height docs according to PR feedback

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

@@ -6,7 +6,7 @@ import 'package:flutter/widgets.dart';
 
 import 'theme.dart';
 
-/// A one logical pixel thick horizontal line, with padding on either
+/// A one device pixel thick horizontal line, with padding on either
 /// side.
 ///
 /// In the material design language, this represents a divider.
@@ -26,19 +26,22 @@ import 'theme.dart';
 class Divider extends StatelessWidget {
   /// Creates a material design divider.
   ///
-  /// The height must be at least 1.0 logical pixels.
+  /// The height must be positive.
   const Divider({
     Key key,
     this.height: 16.0,
     this.indent: 0.0,
     this.color
-  }) : assert(height >= 1.0),
+  }) : assert(height >= 0.0),
        super(key: key);
 
   /// The divider's vertical extent.
   ///
-  /// The divider itself is always drawn as one logical pixel thick horizontal
+  /// The divider itself is always drawn as one device pixel thick horizontal
   /// line that is centered within the height specified by this value.
+  ///
+  /// A divider with a height of 0.0 is always drawn as a line with a height of
+  /// exactly one device pixel, without any padding around it.
   final double height;
 
   /// The amount of empty space to the left of the divider.
@@ -58,19 +61,22 @@ class Divider extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final double bottom = (height ~/ 2.0).toDouble();
-    return new Container(
-      height: 0.0,
-      margin: new EdgeInsets.only(
-        top: height - bottom - 1.0,
-        left: indent,
-        bottom: bottom
+    return new SizedBox(
+      height: height,
+      child: new Center(
+        child: new Container(
+          height: 0.0,
+          margin: new EdgeInsets.only(left: indent),
+          decoration: new BoxDecoration(
+            border: new Border(
+              bottom: new BorderSide(
+                color: color ?? Theme.of(context).dividerColor,
+                width: 0.0,
+              ),
+            ),
+          ),
+        ),
       ),
-      decoration: new BoxDecoration(
-        border: new Border(
-          bottom: new BorderSide(color: color ?? Theme.of(context).dividerColor)
-        )
-      )
     );
   }
 }

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

@@ -82,7 +82,7 @@ class DrawerHeader extends StatelessWidget {
         border: new Border(
           bottom: new BorderSide(
             color: theme.dividerColor,
-            width: 1.0
+            width: 0.0
           )
         )
       ),

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

@@ -294,7 +294,7 @@ class ListTile extends StatelessWidget {
         position: DecorationPosition.foreground,
         decoration: new BoxDecoration(
           border: new Border(
-            bottom: new BorderSide(color: dividerColor),
+            bottom: new BorderSide(color: dividerColor, width: 0.0),
           ),
         ),
         child: tile,

packages/flutter/test/material/divider_test.dart

@@ -4,11 +4,13 @@
 
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import '../rendering/mock_canvas.dart';
 
 void main() {
   testWidgets('Divider control test', (WidgetTester tester) async {
     await tester.pumpWidget(const Center(child: const Divider()));
     final RenderBox box = tester.firstRenderObject(find.byType(Divider));
-    expect(box.size.height, 15.0);
+    expect(box.size.height, 16.0);
+    expect(find.byType(Divider), paints..path(strokeWidth: 0.0));
   });
 }

packages/flutter/test/material/drawer_test.dart

@@ -48,7 +48,7 @@ void main() {
 
     box = tester.renderObject(find.byKey(containerKey));
     expect(box.size.width, equals(drawerWidth - 2 * 16.0));
-    expect(box.size.height, equals(drawerHeight - 2 * 16.0 - 1.0)); // bottom edge
+    expect(box.size.height, equals(drawerHeight - 2 * 16.0));
 
     expect(find.text('header'), findsOneWidget);
   });