Add a stubbed-out implementation of FlowGraphCompiler for ia32 and arm.

runtime/vm/code_generator_x64.h

Index: runtime/vm/code_generator_x64.h
diff --git a/runtime/vm/code_generator_x64.h b/runtime/vm/code_generator_x64.h
index 84ff9560c9e9060be5b1b9f69b8aefc22b10eded..33c73388bd4a1151774c5afa7bafc463b192a004 100644
--- a/runtime/vm/code_generator_x64.h
+++ b/runtime/vm/code_generator_x64.h
@@ -113,6 +113,9 @@ NODE_LIST(DEFINE_VISITOR_FUNCTION)
 
   // Add pc descriptors to code.
   void FinalizePcDescriptors(const Code& code);
+  // Verify assumptions about pc descriptors in debug mode.
+  static void VerifyPcDescriptors(const PcDescriptors& descriptors,
+                                  bool check_ids);
 
   // Allocate and return an arguments descriptor.
   // Let 'num_names' be the length of 'optional_arguments_names'.