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

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

Index: pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
index c23efd500fc60735aa3e8d75a03276333f73ced3..d5d1449537422b0fbfdc0230b71f4dfb65c776d4 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
@@ -895,6 +895,20 @@ class TypeExpression extends Expression {
   }
 }
 
+class Await extends Expression {
+  Expression input;
+
+  Await(this.input);
+
+  accept(ExpressionVisitor visitor) {
+    return visitor.visitAwait(this);
+  }
+
+  accept1(ExpressionVisitor1 visitor, arg) {
+    return visitor.visitAwait(this, arg);
+  }
+}
+
 abstract class ExpressionVisitor<E> {
   E visitExpression(Expression node) => node.accept(this);
   E visitVariableUse(VariableUse node);
@@ -928,6 +942,7 @@ abstract class ExpressionVisitor<E> {
   E visitGetLength(GetLength node);
   E visitGetIndex(GetIndex node);
   E visitSetIndex(SetIndex node);
+  E visitAwait(Await node);
 }
 
 abstract class ExpressionVisitor1<E, A> {
@@ -963,6 +978,7 @@ abstract class ExpressionVisitor1<E, A> {
   E visitGetLength(GetLength node, A arg);
   E visitGetIndex(GetIndex node, A arg);
   E visitSetIndex(SetIndex node, A arg);
+  E visitAwait(Await node, A arg);
 }
 
 abstract class StatementVisitor<S> {
@@ -1196,6 +1212,10 @@ abstract class RecursiveVisitor implements StatementVisitor, ExpressionVisitor {
     visitExpression(node.index);
     visitExpression(node.value);
   }
+
+  visitAwait(Await node) {
+    visitExpression(node.input);
+  }
 }
 
 abstract class Transformer implements ExpressionVisitor<Expression>,
@@ -1438,6 +1458,11 @@ class RecursiveTransformer extends Transformer {
     node.value = visitExpression(node.value);
     return node;
   }
+
+  visitAwait(Await node) {
+    node.input = visitExpression(node.input);
+    return node;
+  }
 }
 
 class FallthroughTarget {