[ddc] Cleanup temporary fix in strSafe

Fix allowed it to accept null but it should never be called with a
nullable value.

Change-Id: I03b5d9b4906e9e4557a4f2720350c0c8176fec53
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/320841
Commit-Queue: Sigmund Cherem <sigmund@google.com>
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Auto-Submit: Nicholas Shahan <nshahan@google.com>

CHANGELOG.md

@@ -79,6 +79,13 @@
   `globalJSObject` is also renamed to `globalContext` and returns the global
   context used in the lowerings.
 
+### Tools
+
+#### Dart Dev Compiler (DDC)
+
+- Applications compiled by DDC will no longer add members to the native
+  JavaScript Object prototype.
+
 ## 3.1.0
 
 ### Libraries

sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart

@@ -1117,8 +1117,6 @@ String str(obj) {
 /// Only called from generated code for string interpolation.
 @notNull
 String strSafe(obj) {
-  // TODO(nshahan) Remove after internal test is fixed b/295486966.
-  if (obj == null) return "null";
   // TODO(40614): Declare `result` as String once non-nullability is sound.
   final result = JS('', '#[#]()', obj, extensionSymbol('toString'));
   if (result is String) return result;