Fixes small bug in a MIPS stub.

R=asiva@google.com

Review URL: https://codereview.chromium.org//16454010

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@23887 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/runtime/vm/stub_code_mips.cc b/runtime/vm/stub_code_mips.cc
index d5219e7..e5cf95c 100644
--- a/runtime/vm/stub_code_mips.cc
+++ b/runtime/vm/stub_code_mips.cc
@@ -1066,7 +1066,7 @@
   // Setup frame, push callee-saved registers.
 
   __ EnterCallRuntimeFrame(0 * kWordSize);
-  __ lw(T0, FieldAddress(CTX, Context::isolate_offset()));
+  __ lw(A0, FieldAddress(CTX, Context::isolate_offset()));
   __ CallRuntime(kStoreBufferBlockProcessRuntimeEntry);
   __ TraceSimMsg("UpdateStoreBufferStub return");
   // Restore callee-saved registers, tear down frame.