Enable await in while and do-while.

runtime/vm/ast.h

Index: runtime/vm/ast.h
diff --git a/runtime/vm/ast.h b/runtime/vm/ast.h
index 95865d083b03d7b71ced72a08e0c5ce42ce02e23..45c4c49f0711bb983c07d5c6087fb847677948b4 100644
--- a/runtime/vm/ast.h
+++ b/runtime/vm/ast.h
@@ -958,7 +958,8 @@ class WhileNode : public AstNode {
     : AstNode(token_pos),
       label_(label),
       condition_(condition),
-      body_(body) {
+      body_(body),
+      condition_preamble_(NULL) {
     ASSERT(label_ != NULL);
     ASSERT(condition_ != NULL);
     ASSERT(body_ != NULL);
@@ -967,8 +968,16 @@ class WhileNode : public AstNode {
   SourceLabel* label() const { return label_; }
   AstNode* condition() const { return condition_; }
   SequenceNode* body() const { return body_; }
+  SequenceNode* condition_preamble() const { return condition_preamble_; }
+
+  void set_condition_preamble(SequenceNode* condition_preamble) {
+    condition_preamble_ = condition_preamble;
+  }
 
   virtual void VisitChildren(AstNodeVisitor* visitor) const {
+    if (condition_preamble() != NULL) {
+      condition_preamble()->Visit(visitor);
+    }
     condition()->Visit(visitor);
     body()->Visit(visitor);
   }
@@ -979,6 +988,7 @@ class WhileNode : public AstNode {
   SourceLabel* label_;
   AstNode* condition_;
   SequenceNode* body_;
+  SequenceNode* condition_preamble_;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(WhileNode);
 };