Add support for WritableRegister policy in the register allocator.

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
diff --git a/runtime/vm/flow_graph_compiler.cc b/runtime/vm/flow_graph_compiler.cc
index 7195d927cab32e51a16b3f5023a26ad459b0c66d..07aad654b101e48c4b6ee52c02e1cdb71806ec4e 100644
--- a/runtime/vm/flow_graph_compiler.cc
+++ b/runtime/vm/flow_graph_compiler.cc
@@ -655,7 +655,8 @@ void FlowGraphCompiler::AllocateRegistersLocally(Instruction* instr) {
     if (loc.IsRegister()) {
       reg = loc.reg();
     } else if (loc.IsUnallocated()) {
-      ASSERT(loc.policy() == Location::kRequiresRegister);
+      ASSERT((loc.policy() == Location::kRequiresRegister) ||
+             (loc.policy() == Location::kWritableRegister));
       reg = AllocateFreeRegister(blocked_registers);
       locs->set_in(i, Location::RegisterLocation(reg));
     }
@@ -684,6 +685,7 @@ void FlowGraphCompiler::AllocateRegistersLocally(Instruction* instr) {
       case Location::kAny:
       case Location::kPrefersRegister:
       case Location::kRequiresRegister:
+      case Location::kWritableRegister:
         result_location = Location::RegisterLocation(
             AllocateFreeRegister(blocked_registers));
         break;