Allow using native JS []= on typed data lists.

sdk/lib/_internal/compiler/implementation/ssa/types.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/types.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/types.dart b/sdk/lib/_internal/compiler/implementation/ssa/types.dart
index 0cc63f9f9a1038b4f4a0497c991731cc7a4a3129..e6a90448d03f18e4d4a77746e5f89787ce3062fa 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/types.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/types.dart
@@ -181,7 +181,6 @@ abstract class HType {
   bool isInteger() => false;
   bool isDouble() => false;
   bool isString() => false;
-  bool isIndexablePrimitive() => false;
   bool isFixedArray() => false;
   bool isReadableArray() => false;
   bool isMutableArray() => false;
@@ -195,6 +194,20 @@ abstract class HType {
   bool isStringOrNull() => false;
   bool isPrimitiveOrNull() => false;
 
+  // TODO(kasperl): Get rid of this one.
+  bool isIndexablePrimitive() => false;
+
+  bool isIndexable(Compiler compiler) =>
+      implementsInterface(compiler.backend.jsIndexableClass, compiler);
+  bool isMutableIndexable(Compiler compiler) =>
+      implementsInterface(compiler.backend.jsMutableIndexableClass, compiler);
+
+  bool implementsInterface(ClassElement interfaceElement, Compiler compiler) {
+    DartType interfaceType = interfaceElement.computeType(compiler);
+    TypeMask mask = new TypeMask.subtype(interfaceType);
+    return mask == mask.union(computeMask(compiler), compiler);
+  }
+
   bool canBeNull() => false;
   bool canBePrimitive(Compiler compiler) => false;
   bool canBePrimitiveNumber(Compiler compiler) => false;