Dart2js CPS: Implement 'await for'.

pkg/compiler/lib/src/tree_ir/optimization/pull_into_initializers.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/pull_into_initializers.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/pull_into_initializers.dart b/pkg/compiler/lib/src/tree_ir/optimization/pull_into_initializers.dart
index 3785f7802f8055d450bd6d44bcd8c8490c550207..8840c8ea0c5951d7bb6b76b7471df028d8a7f7bb 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/pull_into_initializers.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/pull_into_initializers.dart
@@ -265,6 +265,12 @@ class PullIntoInitializers extends RecursiveTransformer
     return node;
   }
 
+  Expression visitAwait(Await node) {
+    super.visitAwait(node);
+    ++impureCounter;
+    return node;
+  }
+
   Expression visitConditional(Conditional node) {
     node.condition = visitExpression(node.condition);
     // Visit the branches to detect impure subexpressions, but do not pull