Reduce the number of captured variables in async code, by only capturing local

runtime/vm/ast_transformer.cc

Index: runtime/vm/ast_transformer.cc
diff --git a/runtime/vm/ast_transformer.cc b/runtime/vm/ast_transformer.cc
index 11cb61cabb661db8f66a1726ff8b1e2bf130c74a..276c8d73dd081fb80f1d35c8abcec3ecff08400a 100644
--- a/runtime/vm/ast_transformer.cc
+++ b/runtime/vm/ast_transformer.cc
@@ -62,7 +62,7 @@ AstNode* AwaitTransformer::Transform(AstNode* expr) {
 
 
 LocalVariable* AwaitTransformer::EnsureCurrentTempVar() {
-  const char* await_temp_prefix = ":await_temp_var_";
+  const char* await_temp_prefix = Symbols::AwaitTempVarPrefix().ToCString();

[hausner, 2015/09/02 22:07:55]

Does it make sense to define this string constant as a symbol, yet the only way
we use it is to convert it to a C string to make another symbol?

You could do this instead:

symbol = String::ZoneHandle(String::NewFormatted("%d", temp_cnt_));
symbol = Symbol::FromConcat(Symbols::AwaitTempVarPrefix(), symbol);

[regis, 2015/09/03 01:28:03]

Done.

[srdjan, 2015/09/03 01:50:47]

This is problematic as it allocates a String in the new space.

[regis, 2015/09/03 16:48:38]

The previous version also allocates a new string, but does not hold onto it
either. Shouldn't the methods of Symbols copy the input from new space to old
space before returning a new Symbol?
Or maybe I do not understand the problem you are mentioning. 

   const String& cnt_str = String::ZoneHandle(
       Z, String::NewFormatted("%s%d", await_temp_prefix, temp_cnt_));
   const String& symbol = String::ZoneHandle(Z, Symbols::New(cnt_str));
@@ -136,8 +136,8 @@ void AwaitTransformer::VisitAwaitNode(AwaitNode* node) {
   AstNode* transformed_expr = Transform(node->expr());
   LocalVariable* await_temp = AddToPreambleNewTempVar(transformed_expr);
 
-  AwaitMarkerNode* await_marker = new (Z) AwaitMarkerNode();
-  await_marker->set_scope(preamble_->scope());
+  AwaitMarkerNode* await_marker =
+      new (Z) AwaitMarkerNode(function_top_, node->scope());
   preamble_->Add(await_marker);
 
   // :result_param = _awaitHelper(