Rewrite for-in loop as indexing loop for JavaScript indexable arrays

sdk/lib/_internal/compiler/js_lib/js_helper.dart

 // Copyright (c) 2013, 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.
 
 library _js_helper;
 
 import 'dart:_async_await_error_codes' as async_error_codes;
 
 import 'dart:_js_embedded_names' show
     DEFERRED_LIBRARY_URIS,
@@ skipping 1417 matching lines @@
 }
 
 throwRuntimeError(message) {
   throw new RuntimeError(message);
 }
 
 throwAbstractClassInstantiationError(className) {
   throw new AbstractClassInstantiationError(className);
 }
 
+// This is used in open coded for-in loops on arrays.
+//
+//     checkConcurrentModificationError(a.length == startLength, a)
+//
+// is replaced in codegen by:
+//
+//     a.length == startLength || throwConcurrentModificationError(a)
+//
+// TODO(sra): We would like to annotate this as @NoSideEffects() so that loops
+// with no other effects can recognize that the array length does not
+// change. However, in the usual case where the loop does have other effects,
+// that causes the length in the loop condition to be phi(startLength,a.length),
+// which causes confusion in range analysis and the insertion of a bounds check.
+@NoInline()
+checkConcurrentModificationError(sameLength, collection) {
+  if (true != sameLength) {
+    throwConcurrentModificationError(collection);
+  }
+}
+
+@NoInline()
+throwConcurrentModificationError(collection) {
+  throw new ConcurrentModificationError(collection);
+}
 
 /**
  * Helper class for building patterns recognizing native type errors.
  */
 class TypeErrorDecoder {
   // Field names are private to help tree-shaking.
 
   /// A regular expression which matches is matched against an error message.
   final String _pattern;
 
@@ skipping 2528 matching lines @@
   // This is a function that will return a helper function that does the
   // iteration of the sync*.
   //
   // Each invocation should give a body with fresh state.
   final dynamic /* js function */ _outerHelper;
 
   SyncStarIterable(this._outerHelper);
 
   Iterator get iterator => new SyncStarIterator(JS('', '#()', _outerHelper));
 }