Let CSP-mode-flag be available as a foreign_helper constant.

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 1b33108bd3b0a86bad1e98c4c6d815962d61dfd8..a4a109e60eb150bf9e310f1423646d0e8420d696 100644
--- a/sdk/lib/_internal/compiler/js_lib/js_helper.dart
+++ b/sdk/lib/_internal/compiler/js_lib/js_helper.dart
@@ -2259,7 +2259,7 @@ abstract class Closure implements Function {
     }
   }
 
-  static bool get isCsp => JS('bool', 'typeof dart_precompiled == "function"');
+  static bool get isCsp => JS_GET_FLAG("USE_CONTENT_SECURITY_POLICY");
 
   static forwardCallTo(receiver, function, bool isIntercepted) {
     if (isIntercepted) return forwardInterceptedCallTo(receiver, function);
@@ -2376,7 +2376,7 @@ abstract class Closure implements Function {
     String receiverField = BoundClosure.receiverFieldName();
     String stubName = JS('String|Null', '#.\$stubName', function);
     int arity = JS('int', '#.length', function);
-    bool isCsp = JS('bool', 'typeof dart_precompiled == "function"');
+    bool isCsp = JS_GET_FLAG("USE_CONTENT_SECURITY_POLICY");
     var lookedUpFunction = JS("", "#[#]", receiver, stubName);
     // The receiver[stubName] may not be equal to the function if we try to
     // forward to a super-method. Especially when we create a bound closure