Specs: rename sky:core to dart:sky

sky/specs/markup.md

 Sky Markup: Syntax
 ==================
 
 A Sky file must consist of the following components:
 
  1. If the file is intended to be a top-level Sky application, the
     string "``#!mojo mojo:sky``" followed by a U+0020, U+000A or
     U+000D character.
 
     If the file is intended to be a module, then the string "SKY", a
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 Sky Markup: Elements
 ====================
 
 The Sky language consists of very few elements, since it is expected
 that everything of note would be provided by frameworks.
 
 The following elements are implicitly registered by default, even if
 you haven't imported anything. You can get to their constructors if
-you import sky:core (basically, sky:core is always imported by defaul;
+you import dart:sky (basically, dart:sky is always imported by defaul;
 it's the runtime library). None of these elements have shadow trees.
 
 ``<import src="foo.sky">``
  - Downloads and imports foo.sky in the background.
 
 ``<import src="foo.sky" as="foo">``
  - Downloads and imports foo.sky in the background, using "foo" as its
    local name (see ``<script>``).
 
 ``<template>``
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 The following attributes are available on all elements:
 
 * ``id=""`` (any value)
 * ``class=""`` (any value, space-separated)
 * ``style=""`` (declaration part of a Sky style rule)
 * ``lang=""`` (language code)
 * ``dir=""`` (ltr or rtl only)
 * ``contenteditable=""`` (subject to future developments)
 * ``tabindex=""`` (subject to future developments)