Optimize length access on all JSIndexable things.

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

Index: sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
index de43ed6a51b660f4c0164575d27f069e13e4a1d5..ce24d01a7258989997626ad3831062e093a305b4 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/backend.dart
@@ -656,8 +656,7 @@ class JavaScriptBackend extends Backend {
   ClassElement jsFixedArrayClass;
   ClassElement jsExtendableArrayClass;
 
-  Element jsArrayLength;
-  Element jsStringLength;
+  Element jsIndexableLength;
   Element jsArrayRemoveLast;
   Element jsArrayAdd;
   Element jsStringSplit;
@@ -910,25 +909,21 @@ class JavaScriptBackend extends Backend {
       jsExtendableArrayClass.ensureResolved(compiler);
     }
 
-    jsArrayClass.ensureResolved(compiler);
-    jsArrayLength = compiler.lookupElementIn(
-        jsArrayClass, const SourceString('length'));
-    if (jsArrayLength != null && jsArrayLength.isAbstractField()) {
-      AbstractFieldElement element = jsArrayLength;
-      jsArrayLength = element.getter;
+    jsIndexableClass.ensureResolved(compiler);
+    jsIndexableLength = compiler.lookupElementIn(
+        jsIndexableClass, const SourceString('length'));
+    if (jsIndexableLength != null && jsIndexableLength.isAbstractField()) {
+      AbstractFieldElement element = jsIndexableLength;
+      jsIndexableLength = element.getter;
     }
+
+    jsArrayClass.ensureResolved(compiler);
     jsArrayRemoveLast = compiler.lookupElementIn(
         jsArrayClass, const SourceString('removeLast'));
     jsArrayAdd = compiler.lookupElementIn(
         jsArrayClass, const SourceString('add'));
 
     jsStringClass.ensureResolved(compiler);
-    jsStringLength = compiler.lookupElementIn(
-        jsStringClass, const SourceString('length'));
-    if (jsStringLength != null && jsStringLength.isAbstractField()) {
-      AbstractFieldElement element = jsStringLength;
-      jsStringLength = element.getter;
-    }
     jsStringSplit = compiler.lookupElementIn(
         jsStringClass, const SourceString('split'));
     jsStringConcat = compiler.lookupElementIn(