Added runtime warnings and notes in the comments

resolves https://github.com/dart-lang/core-elements/issues/142
how about we do these checks by analyzing the actual contents of the `<template>` node above (when we checked for the template to be non null)? I imagine something like:

```dart
if (this.template.content.firstElementChild == null) {
 ...
}
if (this.template.content.childNodes.length > 1) {
 ...
}

```