CTX and TMP registers may not be allocated.

runtime/vm/locations.cc

Index: runtime/vm/locations.cc
===================================================================
--- runtime/vm/locations.cc	(revision 7882)
+++ runtime/vm/locations.cc	(working copy)
@@ -25,7 +25,7 @@
 void LocationSummary::AllocateRegisters() {
   EmbeddedArray<bool, kNumberOfCpuRegisters> blocked_registers;
 
-  // Mark all registers free.
+  // Mark all available registers free.
   for (intptr_t i = 0; i < kNumberOfCpuRegisters; i++) {
     blocked_registers[i] = false;
   }
@@ -39,6 +39,12 @@
     }
   }
 
+  // Do not allocate known registers.
+  blocked_registers[CTX] = true;
+  if (TMP != kNoRegister) {
+    blocked_registers[TMP] = true;
+  }
+
   // Allocate all unallocated input locations.
   for (intptr_t i = 0; i < count(); i++) {
     Location loc = in(i);