Don't allow to iterate over strings.

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

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of ssa;
 
 class SsaFunctionCompiler implements FunctionCompiler {
   final SsaCodeGeneratorTask generator;
   final SsaBuilderTask builder;
   final SsaOptimizerTask optimizer;
@@ skipping 7517 matching lines @@
     // This scheme recognizes for-in on direct lists.  It does not recognize all
     // uses of ArrayIterator.  They still occur when the receiver is an Iterable
     // with a `get iterator` method that delegate to another Iterable and the
     // method is inlined.  We would require full scalar replacement in that
     // case.
 
     Selector selector = Selectors.iterator;
     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);
   }
 
   buildSyncForInIterator(ast.SyncForIn node) {
     // Generate a structure equivalent to:
     //   Iterator<E> $iter = <iterable>.iterator;
     //   while ($iter.moveNext()) {
     //     <declaredIdentifier> = $iter.current;
@@ skipping 1687 matching lines @@
     if (unaliased is TypedefType) throw 'unable to unalias $type';
     unaliased.accept(this, builder);
   }
 
   void visitDynamicType(DynamicType type, SsaBuilder builder) {
     JavaScriptBackend backend = builder.compiler.backend;
     ClassElement cls = backend.helpers.DynamicRuntimeType;
     builder.push(new HDynamicType(type, new TypeMask.exact(cls, classWorld)));
   }
 }