Fix gcc 4.9 build errors.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
diff --git a/runtime/vm/compiler.cc b/runtime/vm/compiler.cc
index 55afac151ad4bded0838a3123d0c5a8ec8760e1a..4e7fc1cce9516e0123200ffb0fd2ad0d149aa435 100644
--- a/runtime/vm/compiler.cc
+++ b/runtime/vm/compiler.cc
@@ -1494,9 +1494,7 @@ void Compiler::ComputeLocalVarDescriptors(const Code& code) {
   const Function& function = Function::Handle(code.function());
   ParsedFunction* parsed_function = new ParsedFunction(
       Thread::Current(), Function::ZoneHandle(function.raw()));
-  LocalVarDescriptors& var_descs =
-      LocalVarDescriptors::Handle(code.var_descriptors());
-  ASSERT(var_descs.IsNull());
+  ASSERT(code.var_descriptors() == Object::null());
   // IsIrregexpFunction have eager var descriptors generation.
   ASSERT(!function.IsIrregexpFunction());
   // In background compilation, parser can produce 'errors": bailouts
@@ -1505,8 +1503,8 @@ void Compiler::ComputeLocalVarDescriptors(const Code& code) {
   if (setjmp(*jump.Set()) == 0) {
     Parser::ParseFunction(parsed_function);
     parsed_function->AllocateVariables();
-    var_descs = parsed_function->node_sequence()->scope()->
-        GetVarDescriptors(function);
+    const LocalVarDescriptors& var_descs = LocalVarDescriptors::Handle(
+        parsed_function->node_sequence()->scope()->GetVarDescriptors(function));
     ASSERT(!var_descs.IsNull());
     code.set_var_descriptors(var_descs);
   } else {