Fix a couple type bugs in dart html and what seems like strange type choices in dart:core

sdk/lib/_internal/compiler/js_lib/js_array.dart

Index: sdk/lib/_internal/compiler/js_lib/js_array.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_array.dart b/sdk/lib/_internal/compiler/js_lib/js_array.dart
index c816907e19338f524b5f77dc337a318019ad7d16..615ed61fbd31e254f2294e5553171d3104e14e7d 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_array.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_array.dart
@@ -213,11 +213,11 @@ class JSArray<E> extends Interceptor implements List<E>, JSIndexable {
     return IterableMixinWorkaround.fold(this, initialValue, combine);
   }
 
-  dynamic firstWhere(bool test(E value), {Object orElse()}) {
+  E firstWhere(bool test(E value), {E orElse()}) {
     return IterableMixinWorkaround.firstWhere(this, test, orElse);
   }
 
-  dynamic lastWhere(bool test(E value), {Object orElse()}) {
+  E lastWhere(bool test(E value), {E orElse()}) {
     return IterableMixinWorkaround.lastWhereList(this, test, orElse);
   }