Check that environments assigned via AssignEnvironment are actually used.

src/lithium-codegen.cc

Index: src/lithium-codegen.cc
diff --git a/src/lithium-codegen.cc b/src/lithium-codegen.cc
index be0ff8371afa51ec664d61a3056d2da13ae7ff79..93ae3101a7779e50b21fe4e542334974bc75758e 100644
--- a/src/lithium-codegen.cc
+++ b/src/lithium-codegen.cc
@@ -122,6 +122,26 @@ bool LCodeGenBase::GenerateBody() {
 }
 
 
+void LCodeGenBase::CheckEnvironmentUsage() {
+#ifdef DEBUG
+  bool live_block = true;
+  for (int i = 0; i < instructions_->length(); i++) {
+    LInstruction* instr = instructions_->at(i);
+    if (instr->IsLabel()) live_block = !LLabel::cast(instr)->HasReplacement();
+    if (live_block &&
+        instr->hydrogen_value()->block()->IsReachable() &&
+        instr->HasEnvironment() &&
+        !instr->environment()->has_been_used()) {
+      FunctionLiteral* lit = info_->function();
+      V8_Fatal(__FILE__, __LINE__, "unused environment in %s <@%d,#%d> %s\n",
+               lit == NULL ? "<UNKNOWN>" : lit->name()->ToCString().get(),
+               i, instr->hydrogen_value()->id(), instr->Mnemonic());
+    }
+  }
+#endif
+}
+
+
 void LCodeGenBase::Comment(const char* format, ...) {
   if (!FLAG_code_comments) return;
   char buffer[4 * KB];