Add errors for declarations which conflict with catch parameters.

src/ast/scopes.h

Index: src/ast/scopes.h
diff --git a/src/ast/scopes.h b/src/ast/scopes.h
index e420cfe053ab4eb616020747c54d23f592715424..8c8a833168792ae3828ee344a5cd723948ec8fb6 100644
--- a/src/ast/scopes.h
+++ b/src/ast/scopes.h
@@ -490,6 +490,12 @@ class Scope: public ZoneObject {
   // The ModuleDescriptor for this scope; only for module scopes.
   ModuleDescriptor* module() const { return module_descriptor_; }
 
+  AstRawString* catch_variable_name() const {
+    DCHECK(is_catch_scope());
+    DCHECK(num_var_or_const() == 1);
+    return static_cast<AstRawString*>(variables_.Start()->key);
+  }
+
   // ---------------------------------------------------------------------------
   // Variable allocation.