Specs: move findId() to TreeScope since it wouldn't be O(1) on Element

sky/specs/apis.md

 APIS
 ====
 
 The Sky core API
 ----------------
 
 ```javascript
 module 'sky:core' {
 
   // EVENTS
@@ skipping 59 matching lines @@
     readonly attribute ChildNode? firstChild; // O(1)
     readonly attribute ChildNode? lastChild; // O(1)
     
     // Returns a new Array every time.
     Array<ChildNode> getChildNodes(); // O(N) in number of child nodes
     Array<Element> getChildElements(); // O(N) in number of child nodes // TODO(ianh): might not be necessary if we have the parser drop unnecessary whitespace text nodes
 
     void append(ChildArgument... nodes); // O(N) in number of arguments plus all their descendants
     void prepend(ChildArgument... nodes); // O(N) in number of arguments plus all their descendants
     void replaceChildrenWith(ChildArgument... nodes); // O(N) in number of descendants plus arguments plus all their descendants
-    
-    Element? findId(String id); // O(1)
   }
 
   interface Attr {
     constructor (String name, String value); // O(1)
     readonly attribute String name; // O(1)
     readonly attribute String value; // O(1)
   }
 
   interface Element : ParentNode {
     readonly attribute String tagName; // O(1)
@@ skipping 30 matching lines @@
     virtual void valueChangeCallback(String? oldValue, String? newValue); // noop
   }
 
   interface DocumentFragment : ParentNode {
     constructor (ChildArguments... nodes); // O(N) in number of arguments plus all their descendants
   }
 
   abstract interface TreeScope : ParentNode {
     readonly attribute Document? ownerDocument; // O(1)
     readonly attribute TreeScope? parentScope; // O(1)
+
+    Element? findId(String id); // O(1)
   }
 
   interface ShadowRoot : TreeScope {
     constructor (ChildArguments... nodes); // O(N) in number of arguments plus all their descendants
     readonly attribute Element? host; // O(1)
     readonly attribute ShadowRoot? olderShadowRoot; // O(1)
     void removeShadowRoot(); // O(N) in descendants
   }
 
   interface Document : TreeScope {
@@ skipping 165 matching lines @@
 magical imports:
   the core mojo fabric JS API   sky:mojo:fabric:core
   the asyncWait/cancelWait mojo fabric JS API (interface to IPC thread)  sky:mojo:fabric:ipc
   the mojom for the shell, proxying through C++ so that the shell pipe isn't exposed  sky:mojo:shell
   the sky API  sky:core
 ```
 
 TODO(ianh): determine if we want to separate the "this" from the
 Document, especially for Modules, so that exposing a module's element
 doesn't expose the module's exports attribute.