Adds support for debugger API on MIPS.

runtime/vm/stack_frame_mips.cc

Index: runtime/vm/stack_frame_mips.cc
===================================================================
--- runtime/vm/stack_frame_mips.cc	(revision 21950)
+++ runtime/vm/stack_frame_mips.cc	(working copy)
@@ -11,6 +11,7 @@
 
 // The constant kExitLinkOffsetInEntryFrame must be kept in sync with the
 // code in the InvokeDartCode stub.
+static const int kSavedContextOffsetInEntryFrame = -11 * kWordSize;
 static const int kExitLinkOffsetInEntryFrame = -10 * kWordSize;
 static const int kPcAddressOffsetFromSp = -2 * kWordSize;
 static const int kEntrypointMarkerOffsetFromFp = 2 * kWordSize;
@@ -43,8 +44,7 @@
 
 
 intptr_t EntryFrame::SavedContextOffset() const {
-  UNIMPLEMENTED();
-  return 0;
+  return kSavedContextOffsetInEntryFrame;
 }