Add a few built-in matchers for HttpServer

[justinfagnani]

It'd be very useful to have a few common matcher functions included with the HttpServer:

Matching on URL is common, so these two would be useful:

• Exact URL match: url("/my/url")
• Regex on URL: pattern("/resource/(\d*)")

[sethladd]

cc @sgjesse.
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[sgjesse]

As the simple matchers can be written as one-liners I don't an immediate need for this.

HttpServer server = ...

// Prefix matching.
server.addHandler((request) => request.path.startsWith("/xxx/yyy/"), ...);

// RegExp matching.
server.addHandler((request) => new RegExp("^/xxx").hasMatch(request.path), ...);

// Exact match.
server.addHandler((request) => request.path == "/yyy.dat", ...);

// Postfix match.
server.addHandler((request) => request.path.endsWith(".dat"), ...);

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[madsager]

I agree with Søren's comment. These are so simple to define yourself that I don't think they belong in the dart:io library.

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[kevmoo]

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