Add ObjectPool tag for external labels. Use LoadExternalLabel for native calls...

runtime/vm/intermediate_language_x64.cc

Index: runtime/vm/intermediate_language_x64.cc
diff --git a/runtime/vm/intermediate_language_x64.cc b/runtime/vm/intermediate_language_x64.cc
index 84afa6fe150a94d6e6dd77e19fa61203df4d6e92..6c150fd3e0ed53271e33626cf6c4b08dd4f85aff 100644
--- a/runtime/vm/intermediate_language_x64.cc
+++ b/runtime/vm/intermediate_language_x64.cc
@@ -781,8 +781,8 @@ void NativeCallInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   } else {
     __ leaq(RAX, Address(RBP, kFirstLocalSlotFromFp * kWordSize));
   }
-  __ LoadImmediate(
-      RBX, Immediate(reinterpret_cast<uword>(native_c_function())));
+  ExternalLabel label(reinterpret_cast<uword>(native_c_function()));
+  __ LoadExternalLabel(RBX, &label, kNotPatchable);
   __ LoadImmediate(R10, Immediate(argc_tag));
   const StubEntry& stub_entry = (is_bootstrap_native() || is_leaf_call) ?
       *StubCode::CallBootstrapCFunction_entry() :
@@ -822,8 +822,9 @@ void StringFromCharCodeInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
   ASSERT(compiler->is_optimizing());
   Register char_code = locs()->in(0).reg();
   Register result = locs()->out(0).reg();
-  __ LoadImmediate(result,
-      Immediate(reinterpret_cast<uword>(Symbols::PredefinedAddress())));
+
+  ExternalLabel label(reinterpret_cast<uword>(Symbols::PredefinedAddress()));
+  __ LoadExternalLabel(result, &label, kNotPatchable);
   __ movq(result, Address(result,
                           char_code,
                           TIMES_HALF_WORD_SIZE,  // Char code is a smi.