dart2js: Introduce HGetLength to replace strange use of HFieldGet

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

Index: pkg/compiler/lib/src/ssa/builder.dart
diff --git a/pkg/compiler/lib/src/ssa/builder.dart b/pkg/compiler/lib/src/ssa/builder.dart
index ec83c92a50d1665705d904cb21b480c6788bd783..17fa53311dccbcec414627427e4bb39eefcbdcf6 100644
--- a/pkg/compiler/lib/src/ssa/builder.dart
+++ b/pkg/compiler/lib/src/ssa/builder.dart
@@ -5482,9 +5482,7 @@ class SsaBuilder extends ast.Visitor
     HInstruction originalLength = null; // Set for growable lists.
 
     HInstruction buildGetLength() {
-      MemberElement lengthElement = helpers.jsIndexableLength;
-      HFieldGet result = new HFieldGet(
-          lengthElement, array, commonMasks.positiveIntType,
+      HInstruction result = new HGetLength(array, commonMasks.positiveIntType,
           isAssignable: !isFixed);
       add(result);
       return result;