dart2js cps: Support async/await by rewriting the JavaScript AST.

pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart

Index: pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
diff --git a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
index ac1d1433f0622eaee7a73ffc8f8601d0f1bde5e2..f39f57be8b42f39d6f694dd93524d52e6f81cdb5 100644
--- a/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
+++ b/pkg/compiler/lib/src/cps_ir/shrinking_reductions.dart
@@ -687,6 +687,11 @@ class ParentVisitor extends RecursiveVisitor {
     node.index.parent = node;
     node.value.parent = node;
   }
+
+  processAwait(Await node) {
+    node.continuation.parent = node;
+    node.input.parent = node;
+  }
 }
 
 class _ReductionKind {