Fix scoping async functions.

runtime/vm/ast_transformer.h

Index: runtime/vm/ast_transformer.h
diff --git a/runtime/vm/ast_transformer.h b/runtime/vm/ast_transformer.h
index 6fbd82bec598676dba00701d02493e7082fdbf26..0ec64eed0c799d36c8928908e700aafa44be2ca9 100644
--- a/runtime/vm/ast_transformer.h
+++ b/runtime/vm/ast_transformer.h
@@ -41,18 +41,8 @@ class AwaitTransformer : public AstNodeVisitor {
  public:
   AwaitTransformer(SequenceNode* preamble,
                    const Library& library,
-                   ParsedFunction* const parsed_function)
-    : preamble_(preamble),
-      temp_cnt_(0),
-      library_(library),
-      parsed_function_(parsed_function),
-      isolate_(Isolate::Current()) {
-    // We later on save the continuation context and modify the jump counter
-    // from within the preamble context (in the FlowGraphBuilder). Look up the
-    // needed variables to get their corresponding aliases.
-    preamble->scope()->LookupVariable(Symbols::AwaitContextVar(), false);
-    preamble->scope()->LookupVariable(Symbols::AwaitJumpVar(), false);
-  }
+                   ParsedFunction* const parsed_function,
+                   LocalScope* function_top);
 
 #define DECLARE_VISIT(BaseName)                                                \
   virtual void Visit##BaseName##Node(BaseName##Node* node);
@@ -69,6 +59,8 @@ class AwaitTransformer : public AstNodeVisitor {
   AstNode* LazyTransform(const Token::Kind kind,
                          AstNode* new_left,
                          AstNode* right);
+  LocalScope* ChainNewScope(LocalScope* parent);
+  LocalVariable* GetVariableInScope(LocalScope* scope, const String& symbol);
 
   void NextTempVar() { temp_cnt_++; }
 
@@ -79,6 +71,7 @@ class AwaitTransformer : public AstNodeVisitor {
   AstNode* result_;
   const Library& library_;
   ParsedFunction* const parsed_function_;
+  LocalScope* function_top_;
 
   Isolate* isolate_;