UIP-3014 Warn when key uses default toString impl

With key being dynamic and toString-ing values, it's easy to pass in any object.

It's also easy to pass in objects with non-unique toString implementations, such as the default Object.toString, and not notice that the value won't help when keying.

class Foo {
  final bar;
  Foo(this.bar);
}

..key = new Foo(1) // Uses "Instance of 'Foo'" as key
..key = new Foo(2) // Uses "Instance of 'Foo'" as key

We should check whether values like this are specified (perhaps via a regex, like ^Instance of '[_\$a-zA-Z0-9]+'$) and warn in the console.

Locate the key-handling entry point and the existing console-warning path, then reproduce the two Foo examples from the issue. The work is done when values using Dart's default object toString form trigger a console warning while other key values continue to work.