Disentangle FontSize from Settings

Instead, just store the default font sizes in statics. These statics are
constants for now, but we'll probably make them configurable at some point.

packages/unit/test/engine/paragraph_builder_test_disabled.dart

@@ -8,6 +8,14 @@ void main() {
     builder.addText('Hello');
     Paragraph paragraph = builder.build(new ParagraphStyle());
     expect(paragraph, isNotNull);
+
+    paragraph.minWidth = 0.0;
+    paragraph.maxWidth = 800.0;
+    paragraph.minHeight = 0.0;
+    paragraph.maxHeight = 600.0;
+
     paragraph.layout();
+    expect(paragraph.width, isNonZero);
+    expect(paragraph.height, isNonZero);
   });
 }