dart2js: add builtin 'isSubtype'.

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

Index: sdk/lib/_internal/compiler/js_lib/js_helper.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/js_helper.dart b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
index 0421f365610b92aea8b99935f2183ec95cbb9855..06638615557098b8a4dd667ec2ef8d61e9e6f0de 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -62,7 +62,6 @@ import 'dart:_foreign_helper' show
     JS_NULL_CLASS_NAME,
     JS_OBJECT_CLASS_NAME,
     JS_OPERATOR_AS_PREFIX,
-    JS_OPERATOR_IS_PREFIX,
     JS_SIGNATURE_NAME,
     JS_STRING_CONCAT,
     RAW_DART_FUNCTION_REF;
@@ -146,6 +145,14 @@ Object getRawRuntimeType(Object o) {
   return JS_BUILTIN('', JsBuiltin.rawRuntimeType, o);
 }
 
+/// Returns whether the given [type] is a subtype of [other].
+///
+/// The argument [other] is the name of the other type, as computed by
+/// [runtimeTypeToString].
+bool builtinIsSubtype(type, String other) {
+  return JS_BUILTIN('returns:bool;effects:none;depends:none',
+                    JsBuiltin.isSubtype, other, type);
+}
 
 /// No-op method that is called to inform the compiler that preambles might
 /// be needed when executing the resulting JS file in a command-line