Work around runtime engine deficiencies.

sdk/lib/_internal/compiler/implementation/js_backend/backend.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(revision 26873)
+++ sdk/lib/_internal/compiler/implementation/js_backend/backend.dart	(working copy)
@@ -1591,6 +1591,15 @@
 
     return false;
   }
+
+  bool isTypedArray(TypeMask mask) {
+    // Just checking for [:TypedData:] is not sufficient, as it is an
+    // abstract class any user-defined class can implement. So we also
+    // check for the interface [JavaScriptIndexingBehavior].
+    return compiler.typedDataClass != null
+        && mask.satisfies(compiler.typedDataClass, compiler)
+        && mask.satisfies(jsIndexingBehaviorInterface, compiler);
+  }
 }
 
 /// Records that [type] is used by [user.element].