Add support behind an experimental compiler option to emit design-type metadata for decorated declarations in source.

• Add __metadata helper that depends on a polyfill for a proposed Reflect.metadata decorator.
• Emit calls to __metadata for class and member (property and method) decorators
• For a class declaration, emit the type metadata for constructor parameters of the constructor with a body.
• For a method declaration, emit the type metadata for the member, its parameters, and its return type.
• For an accessor or property declaration, emit the type metadata for the member.
• Add compiler option to enable experimental metadata support.

A few notes on metadata:

• Type metadata uses the metadata key "design:type".
• Parameter type metadata uses the metadata key "design:paramtypes".
• Return type metadata uses the metadata key "design:returntype".