Handle for-in loops

lib/runtime/dart/_interceptors.js

Index: lib/runtime/dart/_interceptors.js
diff --git a/lib/runtime/dart/_interceptors.js b/lib/runtime/dart/_interceptors.js
index 75ff4d9e5d2ea2949bcdf12a3649e363e851550c..c410d949489914b237f3eb5d1f95ee31cca958fd 100644
--- a/lib/runtime/dart/_interceptors.js
+++ b/lib/runtime/dart/_interceptors.js
@@ -862,8 +862,8 @@ var _interceptors;
       let start = 0;
       let length = 1;
       for (let match of pattern.allMatches(this)) {
-        let matchStart = dart.as(dart.dload(match, 'start'), core.int);
-        let matchEnd = dart.as(dart.dload(match, 'end'), core.int);
+        let matchStart = match.start;
+        let matchEnd = match.end;
         length = dart.notNull(matchEnd) - dart.notNull(matchStart);
         if (length == 0 && start == matchStart) {
           continue;