Use inlined double temporary object for unary operation, add tests. Fix disassembler crash.

runtime/vm/compiler.cc

Index: runtime/vm/compiler.cc
===================================================================
--- runtime/vm/compiler.cc	(revision 3659)
+++ runtime/vm/compiler.cc	(working copy)
@@ -182,7 +182,8 @@
     OS::Print("Variable Descriptors for function '%s' {\n", function_fullname);
     const LocalVarDescriptors& var_descriptors =
         LocalVarDescriptors::Handle(code.var_descriptors());
-    intptr_t var_desc_length = var_descriptors.Length();
+    intptr_t var_desc_length =
+        var_descriptors.IsNull() ? 0 : var_descriptors.Length();
     String& var_name = String::Handle();
     for (intptr_t i = 0; i < var_desc_length; i++) {
       var_name = var_descriptors.GetName(i);