fix #30094, assert should work with a function

pkg/dev_compiler/tool/input_sdk/private/js_helper.dart

Index: pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
index 0017ebc9b16877df510e6ab71c831789f1b79945..3212539c5bfe89342af4400e6c14af63be8d5f61 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_helper.dart
@@ -861,15 +861,12 @@ class RuntimeError extends Error {
   String toString() => "RuntimeError: $message";
 }
 
-/**
- * Error thrown when an assert() fails with a message:
- *
- *     assert(false, "Message here");
- */
-class AssertionErrorWithMessage extends AssertionError {
-  final Object _message;
-  AssertionErrorWithMessage(this._message);
-  String toString() => "Assertion failed: ${_message}";
+/// Error thrown by DDC when an `assert()` fails (with or without a message).
+class AssertionErrorImpl extends AssertionError {
+  AssertionErrorImpl(message) : super(message);
+  String toString() =>
+      "Assertion failed: " +
+      (message != null ? Error.safeToString(message) : "is not true");

[Jennifer Messerly, 2017/07/07 23:43:17]

this uses Error.safeToString as dart2js does

VM/dart2js do not appear to have a separate impl type for asserts with messages
compared to other asserts.

 }
 
 /**