[destructuring] Implement parameter pattern matching.

src/scopes.cc

Index: src/scopes.cc
diff --git a/src/scopes.cc b/src/scopes.cc
index 039bea0987a4261432cab324cdd464e0e87939a6..61a75ab16fcf408a0ba79355f4e43d59979564c8 100644
--- a/src/scopes.cc
+++ b/src/scopes.cc
@@ -466,15 +466,22 @@ Variable* Scope::DeclareParameter(const AstRawString* name, VariableMode mode,
                                   bool is_rest, bool* is_duplicate) {
   DCHECK(!already_resolved());
   DCHECK(is_function_scope());
-  Variable* var = variables_.Declare(this, name, mode, Variable::NORMAL,
-                                     kCreatedInitialized);
+
+  Variable* var;
+  if (!name->IsEmpty()) {
+    var = variables_.Declare(this, name, mode, Variable::NORMAL,
+                             kCreatedInitialized);
+    // TODO(wingo): Avoid O(n^2) check.
+    *is_duplicate = IsDeclaredParameter(name);
+  } else {
+    var = new (zone())
+        Variable(this, name, TEMPORARY, Variable::NORMAL, kCreatedInitialized);
+  }
   if (is_rest) {
     DCHECK_NULL(rest_parameter_);
     rest_parameter_ = var;
     rest_index_ = num_parameters();
   }
-  // TODO(wingo): Avoid O(n^2) check.
-  *is_duplicate = IsDeclaredParameter(name);
   params_.Add(var, zone());
   return var;
 }