Make hasNext a getter instead of a method.

lib/compiler/implementation/js_backend/constant_emitter.dart

Index: lib/compiler/implementation/js_backend/constant_emitter.dart
diff --git a/lib/compiler/implementation/js_backend/constant_emitter.dart b/lib/compiler/implementation/js_backend/constant_emitter.dart
index 792cf27a874d228d068a08ca1bb2d2531c4bb2fe..0ccf3d4d971efcb245eebeadc869fc4dbfd23293 100644
--- a/lib/compiler/implementation/js_backend/constant_emitter.dart
+++ b/lib/compiler/implementation/js_backend/constant_emitter.dart
@@ -91,7 +91,7 @@ class ConstantEmitter implements ConstantVisitor {
                           CodeBuffer buffer,
                           Node diagnosticNode) {
     Iterator<int> iterator = string.iterator();
-    while (iterator.hasNext()) {
+    while (iterator.hasNext) {
       int code = iterator.next();
       if (identical(code, $SQ)) {
         buffer.add(r"\'");