Clean up Handle usage.

Prefer using ConstHandle instead of Handle as function arguments
since you can't assign new references to ConstHandle which helps
prevent bugs.

Changed NullHandle to be a ConstHandle so that you can never modify
it to be a non null reference.

Change-Id: I81cb979f6f8d5b49e5614966a2caf28c3701dd4f
diff --git a/runtime/verifier/method_verifier-inl.h b/runtime/verifier/method_verifier-inl.h
index d4fe106..0f9aeb5 100644
--- a/runtime/verifier/method_verifier-inl.h
+++ b/runtime/verifier/method_verifier-inl.h
@@ -39,11 +39,11 @@
 }
 
 inline mirror::ClassLoader* MethodVerifier::GetClassLoader() {
-  return class_loader_->Get();
+  return class_loader_.Get();
 }
 
 inline mirror::DexCache* MethodVerifier::GetDexCache() {
-  return dex_cache_->Get();
+  return dex_cache_.Get();
 }
 
 inline MethodReference MethodVerifier::GetMethodReference() const {