Use zone allocated C-strings instead of String heap objects to construct signature symbols

runtime/vm/symbols.h

Index: runtime/vm/symbols.h
diff --git a/runtime/vm/symbols.h b/runtime/vm/symbols.h
index 97c6ce10d6aff9f9282d16626a9d6a97c4838fd3..5e2d585559a1dd58d036a683a45a3763b07ceeb2 100644
--- a/runtime/vm/symbols.h
+++ b/runtime/vm/symbols.h
@@ -5,6 +5,7 @@
 #ifndef VM_SYMBOLS_H_
 #define VM_SYMBOLS_H_
 
+#include "vm/growable_array.h"
 #include "vm/object.h"
 #include "vm/snapshot_ids.h"
 
@@ -594,6 +595,8 @@ class Symbols : public AllStatic {
 
   static RawString* FromConcat(const String& str1, const String& str2);
 
+  static RawString* FromConcatAll(const GrowableArray<const String*>& strs);
+
   // Returns char* of predefined symbol.
   static const char* Name(SymbolId symbol);