First cut of mini dart:html.

lib/runtime/dart/_metadata.js

+dart_library.library('dart/_metadata', null, /* Imports */[
+  "dart/_runtime",
+  'dart/core'
+], /* Lazy imports */[
+], function(exports, dart, core) {
+  'use strict';
+  let dartx = dart.dartx;
+  class SupportedBrowser extends core.Object {
+    SupportedBrowser(browserName, minimumVersion) {
+      if (minimumVersion === void 0)
+        minimumVersion = null;
+      this.browserName = browserName;
+      this.minimumVersion = minimumVersion;
+    }
+  }
+  dart.setSignature(SupportedBrowser, {
+    constructors: () => ({SupportedBrowser: [SupportedBrowser, [core.String], [core.String]]})
+  });
+  SupportedBrowser.CHROME = "Chrome";
+  SupportedBrowser.FIREFOX = "Firefox";
+  SupportedBrowser.IE = "Internet Explorer";
+  SupportedBrowser.OPERA = "Opera";
+  SupportedBrowser.SAFARI = "Safari";
+  class Experimental extends core.Object {
+    Experimental() {
+    }
+  }
+  dart.setSignature(Experimental, {
+    constructors: () => ({Experimental: [Experimental, []]})
+  });
+  class DomName extends core.Object {
+    DomName(name) {
+      this.name = name;
+    }
+  }
+  dart.setSignature(DomName, {
+    constructors: () => ({DomName: [DomName, [core.String]]})
+  });
+  class DocsEditable extends core.Object {
+    DocsEditable() {
+    }
+  }
+  dart.setSignature(DocsEditable, {
+    constructors: () => ({DocsEditable: [DocsEditable, []]})
+  });
+  class Unstable extends core.Object {
+    Unstable() {
+    }
+  }
+  dart.setSignature(Unstable, {
+    constructors: () => ({Unstable: [Unstable, []]})
+  });
+  // Exports:
+  exports.SupportedBrowser = SupportedBrowser;
+  exports.Experimental = Experimental;
+  exports.DomName = DomName;
+  exports.DocsEditable = DocsEditable;
+  exports.Unstable = Unstable;
+});