Don't allow to iterate over strings.

pkg/compiler/lib/src/ssa/builder.dart

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index d06a002bc42b14a616f38d7f691a4fb2a759fb17..85fd58338a7e5e5b627034dbb009549182e4e9ba 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -7535,7 +7535,10 @@ class SsaBuilder extends ast.Visitor
     TypeMask mask = elements.getIteratorTypeMask(node);
 
     ClassWorld classWorld = compiler.world;
-    if (mask != null && mask.satisfies(helpers.jsIndexableClass, classWorld)) {
+    if (mask != null &&
+        mask.satisfies(helpers.jsIndexableClass, classWorld) &&
+        // String is indexable but not iterable.
+        !mask.satisfies(helpers.jsStringClass, classWorld)) {
       return buildSyncForInIndexable(node, mask);
     }
     buildSyncForInIterator(node);