Icon should use an enum rather than an int for size

Material design icons are defined to work at specific sizes: 18, 24, 36, 48.
The current API doesn't reflect that and just takes a size int. If an invalid
size is chosen an error is printed to the console and no icon shows up.

Fixes #1816

engine/src/flutter/examples/address_book/lib/main.dart

@@ -20,7 +20,7 @@ class Field extends StatelessComponent {
     return new Row(<Widget>[
         new Padding(
           padding: const EdgeDims.symmetric(horizontal: 16.0),
-          child: new Icon(type: icon, size: 24)
+          child: new Icon(type: icon)
         ),
         new Flexible(
           child: new Input(
@@ -43,7 +43,7 @@ class AddressBookHome extends StatelessComponent {
 
   Widget buildFloatingActionButton(BuildContext context) {
     return new FloatingActionButton(
-      child: new Icon(type: 'image/photo_camera', size: 24),
+      child: new Icon(type: 'image/photo_camera'),
       backgroundColor: Theme.of(context).accentColor
     );
   }