Reland transformation of async functions.

runtime/vm/raw_object.h

Index: runtime/vm/raw_object.h
diff --git a/runtime/vm/raw_object.h b/runtime/vm/raw_object.h
index cf3ca12bac6b3ef002f9c1fee59d729f16515043..1123ff620eeb2e9a7c7681688aad8d6ed5280f70 100644
--- a/runtime/vm/raw_object.h
+++ b/runtime/vm/raw_object.h
@@ -615,6 +615,11 @@ class RawFunction : public RawObject {
     kInvokeFieldDispatcher,  // invokes a field as a closure.
   };
 
+  enum AsyncModifier {
+    kNoModifier,
+    kAsync,
+  };
+
  private:
   // So that the MarkingVisitor::DetachCode can null out the code fields.
   friend class MarkingVisitor;
@@ -650,7 +655,7 @@ class RawFunction : public RawObject {
   int16_t num_fixed_parameters_;
   int16_t num_optional_parameters_;  // > 0: positional; < 0: named.
   int16_t deoptimization_counter_;
-  uint16_t kind_tag_;  // See Function::KindTagBits.
+  uint32_t kind_tag_;  // See Function::KindTagBits.
   uint16_t optimized_instruction_count_;
   uint16_t optimized_call_site_count_;
 };