Reapply "Make int a double in argument type checks."

sdk/lib/_internal/compiler/implementation/lib/js_number.dart

Index: sdk/lib/_internal/compiler/implementation/lib/js_number.dart
diff --git a/sdk/lib/_internal/compiler/implementation/lib/js_number.dart b/sdk/lib/_internal/compiler/implementation/lib/js_number.dart
index b6219e718ca6044f7f92462d8b3f98c5bc5e62be..445e927cbc5165db1d81c3c280129aee7f5a54e5 100644
--- a/sdk/lib/_internal/compiler/implementation/lib/js_number.dart
+++ b/sdk/lib/_internal/compiler/implementation/lib/js_number.dart
@@ -264,7 +264,14 @@ class JSNumber extends Interceptor implements num {
   }
 }
 
-class JSInt extends JSNumber implements int {
+/**
+ * The interceptor class for [int]s.
+ *
+ * This class implements double since in JavaScript all numbers are doubles, so
+ * while we want to treat `2.0` as an integer for some operations, its
+ * interceptor should answer `true` to `is double`.
+ */
+class JSInt extends JSNumber implements int, double {
   const JSInt();
 
   bool get isEven => (this & 1) == 0;