Fix app bar padding without leading widget

If there's no leading widget, we just want 8.0 pixels of padding instead of all
the padding to get over to the 72.0 pixel key line.

Fixes #2912

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

@@ -134,16 +134,18 @@ class AppBar extends StatelessWidget {
     }
 
     final List<Widget> toolBarRow = <Widget>[];
-    if (leading != null)
-      toolBarRow.add(leading);
-    toolBarRow.add(
-      new Flexible(
-        child: new Padding(
-          padding: new EdgeInsets.only(left: 24.0),
-          child: title != null ? new DefaultTextStyle(style: centerStyle, child: title) : null
-        )
+    if (leading != null) {
+      toolBarRow.add(new Padding(
+        padding: new EdgeInsets.only(right: 16.0),
+        child: leading
+      ));
+    }
+    toolBarRow.add(new Flexible(
+      child: new Padding(
+        padding: new EdgeInsets.only(left: 8.0),
+        child: title != null ? new DefaultTextStyle(style: centerStyle, child: title) : null
       )
-    );
+    ));
     if (actions != null)
       toolBarRow.addAll(actions);