[es6] implement optional parameters via desugaring (with scoping)

src/globals.h

Index: src/globals.h
diff --git a/src/globals.h b/src/globals.h
index e71314f9c1af9558e6e249e16cdaab7dca870397..ae4f47205006b73bc06e8966d112d99fc1b451ea 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -710,14 +710,15 @@ enum SmiCheckType {
 
 
 enum ScopeType {
-  EVAL_SCOPE,      // The top-level scope for an eval source.
-  FUNCTION_SCOPE,  // The top-level scope for a function.
-  MODULE_SCOPE,    // The scope introduced by a module literal
-  SCRIPT_SCOPE,    // The top-level scope for a script or a top-level eval.
-  CATCH_SCOPE,     // The scope introduced by catch.
-  BLOCK_SCOPE,     // The scope introduced by a new block.
-  WITH_SCOPE,      // The scope introduced by with.
-  ARROW_SCOPE      // The top-level scope for an arrow function literal.
+  EVAL_SCOPE,          // The top-level scope for an eval source.
+  FUNCTION_SCOPE,      // The top-level scope for a function.
+  MODULE_SCOPE,        // The scope introduced by a module literal
+  SCRIPT_SCOPE,        // The top-level scope for a script or a top-level eval.
+  CATCH_SCOPE,         // The scope introduced by catch.
+  BLOCK_SCOPE,         // The scope introduced by a new block.
+  WITH_SCOPE,          // The scope introduced by with.
+  ARROW_SCOPE,         // The top-level scope for an arrow function literal.
+  FUNCTION_BODY_SCOPE  // The body of a function
 };
 
 // The mips architecture prior to revision 5 has inverted encoding for sNaN.