Add support for line-height in TextStyle.

Also, some minor cleanup in TextStyle to make it more readable.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1203253002.

examples/widgets/styled_text.dart

@@ -8,7 +8,6 @@ import 'package:sky/rendering/flex.dart';
 import 'package:sky/rendering/sky_binding.dart';
 import 'package:sky/theme/colors.dart';
 import 'package:sky/theme/theme_data.dart';
-import 'package:sky/theme/typography.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/scaffold.dart';
@@ -43,7 +42,7 @@ HAL: This mission is too important for me to allow you to jeopardize it.''';
   // [["Dave", "Open the pod bay..."] ...]
   List<List<String>> nameLines;
 
-  final TextStyle daveStyle = new TextStyle(color: Indigo[400]);
+  final TextStyle daveStyle = new TextStyle(color: Indigo[400], height: 1.8);
   final TextStyle halStyle = new TextStyle(color: Red[400], fontFamily: "monospace");
   final TextStyle boldStyle = const TextStyle(fontWeight: bold);
   final TextStyle underlineStyle = const TextStyle(