Print 'await' when for is async.

R=asgerf@google.com

Review-Url: https://codereview.chromium.org/2527253002 .

pkg/kernel/lib/text/ast_to_text.dart

@@ -1162,6 +1162,9 @@ class Printer extends Visitor<Null> {
 
   visitForInStatement(ForInStatement node) {
     writeIndentation();
+    if (node.isAsync) {
+      writeSpaced('await');
+    }
     writeSpaced('for');
     writeSymbol('(');
     writeVariableDeclaration(node.variable, useVarKeyword: true);