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

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

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 5784bba24b79215f51d5a284fc2fa9d3c17bc5fc..e2e66d013967cc95894abf5550fb710fb474925a 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -696,7 +696,7 @@ class TransformingVisitor extends LeafVisitor {
     context.body = node;
     node.parent = context;
   }
-  
+
   /// Binds [prim] before [node].
   void insertLetPrim(Expression node, Primitive prim) {
     InteriorNode parent = node.parent;
@@ -2239,6 +2239,12 @@ class TypePropagationVisitor implements Visitor {
   void visitSetIndex(SetIndex node) {
     setValue(node, nonConstant());
   }
+
+  @override
+  visitAwait(Await node) {
+    Continuation continuation = node.continuation.definition;
+    setReachable(continuation);
+  }
 }
 
 /// Represents the abstract value of a primitive value at some point in the