Let's hide double.INFINITY a bit more, by providing cleaner APIs for the cases...

Let's hide double.INFINITY a bit more, by providing cleaner APIs for the cases where we're currently trying to use it.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1209233002.

examples/widgets/styled_text.dart

@@ -63,7 +63,7 @@ HAL: This mission is too important for me to allow you to jeopardize it.''';
 
   Component createSeparator() {
     return new Container(
-      constraints: const BoxConstraints(minWidth: double.INFINITY, maxHeight: 0.0),
+      constraints: const BoxConstraints.expandWidth(maxHeight: 0.0),
       margin: const EdgeDims.symmetric(vertical: 10.0, horizontal: 64.0),
       decoration: const BoxDecoration(
         border: const Border(