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

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index 4ec13ad441624bc15a949ae86de4e3b919fe75fd..77806c00c334282149e35f446964ba6b83a99a65 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -2705,6 +2705,10 @@ class IrBuilder {
         new CallStructure.unnamed(1),
         <ir.Primitive>[value]);
   }
+
+  ir.Node buildAwait(ir.Primitive value) {
+    return _continueWithExpression((k) => new ir.Await(value, k));
+  }
 }