Revert "dart2js: Do not use JSDouble for native behavior return types."

pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart

Index: pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
diff --git a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
index d394bb7c3dbba41ebd3944c6d0d3442792df9e12..b242c98e140d4a3a58feb1fb02e2585b75769ba2 100644
--- a/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
+++ b/pkg/compiler/lib/src/inferrer/simple_types_inferrer.dart
@@ -373,11 +373,9 @@ abstract class InferrerEngine<T, V extends TypeSystem>
         mappedType = types.stringType;
       } else if (type.element == compiler.intClass) {
         mappedType = types.intType;
-      } else if (type.element == compiler.numClass ||
-                 type.element == compiler.doubleClass) {
-        // Note: the backend double class is specifically for non-integer
-        // doubles, and a native behavior returning 'double' does not guarantee
-        // a non-integer return type, so we return the number type for those.
+      } else if (type.element == compiler.doubleClass) {
+        mappedType = types.doubleType;
+      } else if (type.element == compiler.numClass) {
         mappedType = types.numType;
       } else if (type.element == compiler.boolClass) {
         mappedType = types.boolType;