Adds native/leaf runtime call stub and redirection on MIPS.

runtime/vm/flow_graph_compiler_mips.cc

Index: runtime/vm/flow_graph_compiler_mips.cc
===================================================================
--- runtime/vm/flow_graph_compiler_mips.cc	(revision 20794)
+++ runtime/vm/flow_graph_compiler_mips.cc	(working copy)
@@ -252,6 +252,7 @@
   __ EnterDartFrame((StackSize() * kWordSize));
 }
 
+
 // Input parameters:
 //   RA: return address.
 //   SP: address of last argument.
@@ -404,7 +405,9 @@
                                      const ExternalLabel* label,
                                      PcDescriptors::Kind kind,
                                      LocationSummary* locs) {
-  UNIMPLEMENTED();
+  __ BranchLinkPatchable(label);
+  AddCurrentDescriptor(kind, Isolate::kNoDeoptId, token_pos);
+  RecordSafepoint(locs);
 }