With --noopt run unoptimized code through optimizer, more optimizations can be done later.

runtime/vm/report.cc

 // Copyright (c) 2014, 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/report.h"
 
 #include "vm/code_patcher.h"
 #include "vm/exceptions.h"
 #include "vm/flags.h"
 #include "vm/longjump.h"
 #include "vm/object.h"
 #include "vm/stack_frame.h"
 #include "vm/symbols.h"
 
 namespace dart {
 
 DEFINE_FLAG(int, stacktrace_depth_on_warning, 5,
             "Maximal number of stack frames to print after a runtime warning.");
 DEFINE_FLAG(bool, silent_warnings, false, "Silence warnings.");
 DEFINE_FLAG(bool, warn_on_javascript_compatibility, false,
             "Warn on incompatibilities between vm and dart2js.");
 DEFINE_FLAG(bool, warning_as_error, false, "Treat warnings as errors.");
 
+DECLARE_FLAG(bool, always_megamorphic_calls);
 
 RawString* Report::PrependSnippet(Kind kind,
                                   const Script& script,
                                   intptr_t token_pos,
                                   const String& message) {
   const char* message_header;
   switch (kind) {
     case kWarning: message_header = "warning"; break;
     case kJSWarning: message_header = "javascript compatibility warning"; break;
     case kError: message_header = "error"; break;
@@ skipping 137 matching lines @@
   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();
   ICData& ic_data = ICData::Handle();
   if (is_static_native) {
     // Assume an unoptimized static call. Optimization was prevented.
     CodePatcher::GetUnoptimizedStaticCallAt(caller_pc, caller_code, &ic_data);
   } else {
-    // Assume an instance call.
-    CodePatcher::GetInstanceCallAt(caller_pc, caller_code, &ic_data);
+    if (FLAG_always_megamorphic_calls) {
+      Report::JSWarningFromFrame(caller_frame, msg);
+      return;
+    } else {
+      // Assume an instance call.
+      CodePatcher::GetInstanceCallAt(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();
     Report::JSWarningFromFrame(caller_frame, msg);
   }
 }
 
 
@@ skipping 41 matching lines @@
     trace_warning.AddProperty("type", "JSCompatibilityWarning");
     trace_warning.AddProperty("script", script);
     trace_warning.AddProperty("tokenPos", token_pos);
     trace_warning.AddProperty("message", message);
   }
   trace_buffer->Trace(micros, js.ToCString(), true);  // Already escaped.
 }
 
 }  // namespace dart