Add supported checks to the SVG library, and library cleanup.

tests/html/svg_1_test.dart

-library SVG1Test;
-import '../../pkg/unittest/lib/unittest.dart';
-import '../../pkg/unittest/lib/html_config.dart';
-import 'dart:html';
-import 'dart:svg' as svg;
-
-// Test that SVG is present in dart:html API
-
-main() {
-  useHtmlConfiguration();
-
-  var isSvgElement = predicate((x) => x is svg.SvgElement, 'is a SvgElement');
-
-  test('simpleRect', () {
-      var div = new Element.tag('div');
-      document.body.nodes.add(div);
-      div.innerHtml = r'''
-<svg id='svg1' width='200' height='100'>
-<rect id='rect1' x='10' y='20' width='130' height='40' rx='5'fill='blue'></rect>
-</svg>
-
-''';
-
-    var e = document.query('#svg1');
-    expect(e, isNotNull);
-
-    svg.RectElement r = document.query('#rect1');
-    expect(r.x.baseVal.value, 10);
-    expect(r.y.baseVal.value, 20);
-    expect(r.height.baseVal.value, 40);
-    expect(r.width.baseVal.value, 130);
-    expect(r.rx.baseVal.value, 5);
-  });
-
-  test('trailing newline', () {
-    // Ensures that we handle SVG with trailing newlines.
-    var logo = new svg.SvgElement.svg("""
-      <svg xmlns="http://www.w3.org/2000/svg" version="1.1">
-        <path/>
-      </svg>
-      """);
-
-  expect(logo, isSvgElement);
-
-  });
-}