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

pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
index f07a03005ffdc30b24750bc064f0190201e66c81..23deb5fce06f2046205ab9c0e46c739752bb68dd 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_tracer.dart
@@ -555,6 +555,12 @@ class SubexpressionVisitor extends ExpressionVisitor<String> {
     String value = visitExpression(node.value);
     return 'SetIndex($object, $index, $value)';
   }
+
+  @override
+  String visitAwait(Await node) {
+    String value = visitExpression(node.input);
+    return 'Await($value)';
+  }
 }
 
 /**