Add javascript compatibility warnings for strings that are not identical, but

runtime/lib/identical.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/bootstrap_natives.h"
 
+#include "vm/code_patcher.h"
+#include "vm/exceptions.h"
 #include "vm/object.h"
+#include "vm/stack_frame.h"
 
 namespace dart {
 
+DECLARE_FLAG(bool, warn_on_javascript_compatibility);
+
+static void JSWarning(const char* msg) {
+  DartFrameIterator iterator;
+  iterator.NextFrame();  // Skip native call.
+  StackFrame* caller_frame = iterator.NextFrame();
+  ASSERT(caller_frame != NULL);
+  const Code& caller_code = Code::Handle(caller_frame->LookupDartCode());
+  ASSERT(!caller_code.IsNull());
+  const uword caller_pc = caller_frame->pc();
+  // Assume an unoptimized static call. Optimization was prevented.
+  ICData& ic_data = ICData::Handle();
+  CodePatcher::GetUnoptimizedStaticCallAt(caller_pc, caller_code, &ic_data);
+  ASSERT(!ic_data.IsNull());
+  // Report warning only if not already reported at this location.
+  if (!ic_data.IssuedJSWarning()) {
+    ic_data.SetIssuedJSWarning();
+    Exceptions::JSWarning(caller_frame, "%s", msg);
+  }
+}
+
+
 DEFINE_NATIVE_ENTRY(Identical_comparison, 2) {
   GET_NATIVE_ARGUMENT(Instance, a, arguments->NativeArgAt(0));
   GET_NATIVE_ARGUMENT(Instance, b, arguments->NativeArgAt(1));
-  return Bool::Get(a.IsIdenticalTo(b)).raw();
+  const bool is_identical = a.IsIdenticalTo(b);
+  if (FLAG_warn_on_javascript_compatibility) {
+    if (!is_identical && a.IsString() && String::Cast(a).Equals(b)) {
+      JSWarning("strings that are equal are also identical");
+    }
+  }
+  return Bool::Get(is_identical).raw();
 }
 
 }  // namespace dart