Add a basic sky-element custom element

After this CL, you can use <sky-element> to describe custom elements. The
current iteration is very basic and is hardcoded to "example", but its a start.

This CL renames the |init| function to |_init| to prevent importers from
calling it directly. Also, we now pass the <script> element to |_init| to give
some context.

R=ojan@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/950493003

engine/src/flutter/examples/example-element/example-element.sky

@@ -0,0 +1,25 @@
+<!--
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+-->
+<import src="/sky/framework/sky-element.sky" />
+<sky-element>
+<template>
+  <style>
+    div {
+      height: 50px;
+      width: 50px;
+      background-color: green;
+    }
+  </style>
+  <div>Hi</div>
+</template>
+<script>
+@Tagname('example')
+class ExampleElement extends SkyElement {
+}
+
+_init(script) => register(script, ExampleElement);
+</script>
+</sky-element>

engine/src/flutter/examples/example-element/index.sky

@@ -0,0 +1,16 @@
+<!--
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+-->
+<sky>
+  <import src="example-element.sky" />
+  <style>
+    example {
+      border: 1px solid blue;
+      width: 100px;
+      height: 100px;
+    }
+  </style>
+  <example />
+</sky>

engine/src/flutter/tests/modules/resources/multiple-scripts-child.sky

@@ -2,7 +2,7 @@
 <script>
 final String one = 'one';
 bool oneInit = false;
-void init() {
+void _init(_) {
     oneInit = true;
 }
 final conflict = 'one';
@@ -10,7 +10,7 @@ final conflict = 'one';
 <script>
 final String two = 'two';
 bool twoInit = false;
-void init() {
+void _init(_) {
     twoInit = true;
 }
 final conflict = 'two';

engine/src/flutter/tests/modules/resources/with-dart-import.sky

@@ -6,7 +6,7 @@ class TestClass extends base.BaseClass {
 
 bool didInit = false;
 
-void init() {
+void _init(_) {
     didInit = (new TestClass() != null);
 }
 </script>