Add base eval step for map keys while parsing

Keys need to be immutable for the main structure, so we
need to evaluate them while the map is being parsed. We
may defer the map initialization into the eval step too.

Fixes #1169

parser.cpp

@@ -1010,6 +1010,16 @@ namespace Sass {
   {
     To_String to_string(&ctx);
     Expression* key = parse_list();
+    if (String_Quoted* str = dynamic_cast<String_Quoted*>(key)) {
+      if (!str->quote_mark() && !str->is_delayed()) {
+        if (ctx.names_to_colors.count(str->value())) {
+          Color* c = new (ctx.mem) Color(*ctx.names_to_colors[str->value()]);
+          c->pstate(str->pstate());
+          c->disp(str->value());
+          key = c;
+        }
+      }
+    }
 
     // it's not a map so return the lexed value as a list value
     if (!peek< exactly<':'> >())
@@ -1029,6 +1039,16 @@ namespace Sass {
       { break; }
 
       Expression* key = parse_list();
+      if (String_Quoted* str = dynamic_cast<String_Quoted*>(key)) {
+        if (!str->quote_mark() && !str->is_delayed()) {
+          if (ctx.names_to_colors.count(str->value())) {
+            Color* c = new (ctx.mem) Color(*ctx.names_to_colors[str->value()]);
+            c->pstate(str->pstate());
+            c->disp(str->value());
+            key = c;
+          }
+        }
+      }
 
       if (!(lex< exactly<':'> >()))
       { error("invalid syntax", pstate); }