Support Dynamic Proxy

[DartBot]

This issue was originally filed by @tomyeh

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I am looking forward a mechanism similar to Java's dynamic proxy. Without it, it is tedious to intercept a complicated class.

Mixin + implicit class seems possible but the class has to be declared at compile time and there must be a concrete public class available.

[sethladd]

Hi Tom, can you provide a use case for this? What are you trying to do? (Try not to use the words "dynamic proxy" so we can better understand your intent) Thanks!

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Removed Type-Defect label.
Added Type-Enhancement, Area-Library, Triaged labels.

[DartBot]

This comment was originally written by @tomyeh

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Let us say I'd like to extend CSSStyleDeclaration to handle some properties differently. However, there is no implementation available, so I have to implement it from scratch and delegate every method to another except the methods I care:

class Foo implements CSSStyleDeclaration {
   Foo(CSSStyleDeclaration this._css);
   String get animate => _css.animate;
...

But, there are hundreds of methods. It is tedious (and not smart) to do it one by one. (I actually did it by overriding noSuchMethod, but it is right since it ends with non-concrete class)

Let us make the case more complicated. What happens if I'd like to alter some method for a class that we didn't know in compile time? Java dynamic proxy did a great job here. It is worth to have a similar feature.

[DartBot]

This comment was originally written by diego.rocha...@gmail.com

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This issue seems to be the same as #­5641.

[floitschG]

Added Duplicate label.
Marked as being merged into #5641.