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

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
index ba67e956fd69f044ee4b4b58323cd3920567768b..e50c8945adfa29eef715d524b45a25ab712e8710 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_tracer.dart
@@ -382,6 +382,13 @@ class IRTracer extends TracerUtil implements cps_ir.Visitor {
     String value = formatReference(node.value);
     return 'SetIndex $object $index $value';
   }
+
+  @override
+  visitAwait(cps_ir.Await node) {
+    String value = formatReference(node.input);
+    String continuation = formatReference(node.continuation);
+    return 'Await $value $continuation';
+  }
 }
 
 /**
@@ -653,4 +660,9 @@ class BlockCollector implements cps_ir.Visitor {
   visitForeignCode(cps_ir.ForeignCode node) {
     addEdgeToContinuation(node.continuation);
   }
+
+  @override
+  visitAwait(cps_ir.Await node) {
+    unexpectedNode(node);
+  }
 }