Fixes for sunflower

tool/input_sdk/private/js_helper.dart

Index: tool/input_sdk/private/js_helper.dart
diff --git a/tool/input_sdk/private/js_helper.dart b/tool/input_sdk/private/js_helper.dart
index e36c37147609e160a50a147cf2e8dead981811ba..6082b11fee2ec9312887c84375f263c3009d9fcd 100644
--- a/tool/input_sdk/private/js_helper.dart
+++ b/tool/input_sdk/private/js_helper.dart
@@ -582,7 +582,7 @@ class Primitives {
     return JS('int', '#', hash);
   }
 
-  static _throwFormatException(String string) {
+  static Null _throwFormatException(String string) {

[vsm, 2015/05/26 13:40:58]

This feels a bit strange, but see the casts in the corresponding generated JS
file - both were failing.

[Leaf, 2015/05/26 18:19:45]

This is a bit odd.  I don't understand why Null helps with the casts.  Null
isn't bottom, it's a direct subtype of Object, so I would expect this to be
worse (since it's not even castable to anything except object).

[vsm, 2015/05/27 13:37:21]

Ugh, it turns out that we were reporting an error with Null - we were just
suppressing it and forcing the compile anyway.

OTOH, "void" here currently does work - it's treated as bottom by our rules.  We
could use that.

[Leaf, 2015/05/27 22:16:34]

I don't understand this either... :}  I don't think we treat void as bottom.  We
allow void returning functions to be supertypes of functions which return
anything.  But this is the other way around, isn't it?  We're assigning a String
-> void to a String -> int and String -> double.  This requires that String ->
void be a subtype of String -> int/double, which requires void to be a subtype
of int/double, which it isn't.  This seems like it should still be an error?

     throw new FormatException(string);
   }