Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/gc/space/bump_pointer_space.cc b/runtime/gc/space/bump_pointer_space.cc
index 8f42642..0a55b52 100644
--- a/runtime/gc/space/bump_pointer_space.cc
+++ b/runtime/gc/space/bump_pointer_space.cc
@@ -201,8 +201,8 @@
 }
 
 accounting::ContinuousSpaceBitmap::SweepCallback* BumpPointerSpace::GetSweepCallback() {
-  LOG(FATAL) << "Unimplemented";
-  return nullptr;
+  UNIMPLEMENTED(FATAL);
+  UNREACHABLE();
 }
 
 uint64_t BumpPointerSpace::GetBytesAllocated() {
diff --git a/runtime/gc/space/space.cc b/runtime/gc/space/space.cc
index bff28f6..b233805 100644
--- a/runtime/gc/space/space.cc
+++ b/runtime/gc/space/space.cc
@@ -39,33 +39,33 @@
 }
 
 DlMallocSpace* Space::AsDlMallocSpace() {
-  LOG(FATAL) << "Unreachable";
-  return nullptr;
+  UNIMPLEMENTED(FATAL) << "Unreachable";
+  UNREACHABLE();
 }
 
 RosAllocSpace* Space::AsRosAllocSpace() {
-  LOG(FATAL) << "Unreachable";
-  return nullptr;
+  UNIMPLEMENTED(FATAL) << "Unreachable";
+  UNREACHABLE();
 }
 
 ZygoteSpace* Space::AsZygoteSpace() {
-  LOG(FATAL) << "Unreachable";
-  return nullptr;
+  UNIMPLEMENTED(FATAL) << "Unreachable";
+  UNREACHABLE();
 }
 
 BumpPointerSpace* Space::AsBumpPointerSpace() {
-  LOG(FATAL) << "Unreachable";
-  return nullptr;
+  UNIMPLEMENTED(FATAL) << "Unreachable";
+  UNREACHABLE();
 }
 
 AllocSpace* Space::AsAllocSpace() {
-  LOG(FATAL) << "Unimplemented";
-  return nullptr;
+  UNIMPLEMENTED(FATAL) << "Unreachable";
+  UNREACHABLE();
 }
 
 ContinuousMemMapAllocSpace* Space::AsContinuousMemMapAllocSpace() {
-  LOG(FATAL) << "Unimplemented";
-  return nullptr;
+  UNIMPLEMENTED(FATAL) << "Unreachable";
+  UNREACHABLE();
 }
 
 DiscontinuousSpace::DiscontinuousSpace(const std::string& name,
diff --git a/runtime/gc/space/zygote_space.cc b/runtime/gc/space/zygote_space.cc
index 51d84f5..9de0548 100644
--- a/runtime/gc/space/zygote_space.cc
+++ b/runtime/gc/space/zygote_space.cc
@@ -58,7 +58,8 @@
 }
 
 void ZygoteSpace::Clear() {
-  LOG(FATAL) << "Unimplemented";
+  UNIMPLEMENTED(FATAL);
+  UNREACHABLE();
 }
 
 ZygoteSpace::ZygoteSpace(const std::string& name, MemMap* mem_map, size_t objects_allocated)