Implements external array access for mips.

runtime/vm/flow_graph_compiler_mips.cc

Index: runtime/vm/flow_graph_compiler_mips.cc
===================================================================
--- runtime/vm/flow_graph_compiler_mips.cc	(revision 24520)
+++ runtime/vm/flow_graph_compiler_mips.cc	(working copy)
@@ -1676,7 +1676,7 @@
                                                           intptr_t index_scale,
                                                           Register array,
                                                           Register index) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
   return FieldAddress(array, index);
 }
 
@@ -1685,7 +1685,7 @@
     intptr_t index_scale,
     Register array,
     intptr_t index) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
   return FieldAddress(array, index);
 }
 
@@ -1694,7 +1694,7 @@
     intptr_t index_scale,
     Register array,
     Register index) {
-  UNIMPLEMENTED();
+  UNREACHABLE();
   return FieldAddress(array, index);
 }