Pattern match on generated code to find edge counters.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 365c8074b2b0e7be1de221989064002c057e2acf..50c4930838a38795c4f28430b821ce0e61609d78 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1664,6 +1664,12 @@ void JoinEntryInstr::EmitNativeCode(FlowGraphCompiler* compiler) {
 
 
 LocationSummary* TargetEntryInstr::MakeLocationSummary() const {
+  // FlowGraphCompiler::EmitInstructionPrologue is not called for block
+  // entry instructions, so this function is unused.  If it becomes
+  // reachable, note that the deoptimization descriptor in unoptimized code
+  // comes after the point of local register allocation due to pattern
+  // matching the edge counter code backwards (as a code reuse convenience
+  // on some platforms).
   UNREACHABLE();
   return NULL;
 }