cpsir: triage remainder of html test suite, fix a couple.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 88d2b88c86f1089f9d024c72fb6b1f14627fea11..25dce9d907caed58d2853ac2eb9d8007692b7d2f 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -15416,7 +15416,7 @@ class Event extends Interceptor {
   @DomName('Event.currentTarget')
   @DocsEditable()
   @Creates('Null')
-  @Returns('EventTarget|=Object')
+  @Returns('EventTarget|=Object|Null')

[Siggi Cherem (dart-lang), 2016/02/06 02:36:27]

we didn't take advantage of these effects in SSA - so they only showed up as
issues in CPS. 

These changes fix keyboard_event & mouse_event tests

   final dynamic _get_currentTarget;
 
   @DomName('Event.defaultPrevented')
@@ -15451,7 +15451,7 @@ class Event extends Interceptor {
   @DomName('Event.target')
   @DocsEditable()
   @Creates('Node')
-  @Returns('EventTarget|=Object')
+  @Returns('EventTarget|=Object|Null')
   final dynamic _get_target;
 
   @DomName('Event.timeStamp')
@@ -22962,7 +22962,7 @@ class MouseEvent extends UIEvent {
   @DomName('MouseEvent.relatedTarget')
   @DocsEditable()
   @Creates('Node')
-  @Returns('EventTarget|=Object')
+  @Returns('EventTarget|=Object|Null')
   final dynamic _get_relatedTarget;
 
   @JSName('screenX')