Dex2oat support for multiple oat file and image file outputs.

Multiple changes to dex2oat and the runtime to support a --multi-image
option. This generates a separate oat file and image file output for
each dex file input.

Change-Id: Ie1d6f0b8afa8aed5790065b8c2eb177990c60129
diff --git a/runtime/gc/accounting/mod_union_table.cc b/runtime/gc/accounting/mod_union_table.cc
index 8f7bb94..d16afd9 100644
--- a/runtime/gc/accounting/mod_union_table.cc
+++ b/runtime/gc/accounting/mod_union_table.cc
@@ -487,7 +487,9 @@
 
 // Mark all references to the alloc space(s).
 void ModUnionTableCardCache::UpdateAndMarkReferences(MarkObjectVisitor* visitor) {
-  auto* image_space = heap_->GetBootImageSpace();
+  // TODO: Needs better support for multi-images? b/26317072
+  space::ImageSpace* image_space =
+      heap_->GetBootImageSpaces().empty() ? nullptr : heap_->GetBootImageSpaces()[0];
   // If we don't have an image space, just pass in space_ as the immune space. Pass in the same
   // space_ instead of image_space to avoid a null check in ModUnionUpdateObjectReferencesVisitor.
   CardBitVisitor bit_visitor(visitor, space_, image_space != nullptr ? image_space : space_,