dart2js: Introduce HGetLength to replace strange use of HFieldGet

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

 // Copyright (c) 2012, 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.
 
 import '../constant_system_dart.dart';
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
 import '../js_backend/backend_helpers.dart';
 import '../world.dart' show ClosedWorld;
 import 'nodes.dart';
@@ skipping 680 matching lines @@
     }
     if (constantNum.isPositiveInfinity || constantNum.isNegativeInfinity) {
       return info.newUnboundRange();
     }
     if (constantNum.isMinusZero) constantNum = new IntConstantValue(0);
     Value value = info.newIntValue(constantNum.primitiveValue);
     return info.newNormalizedRange(value, value);
   }
 
   Range visitFieldGet(HFieldGet fieldGet) {
-    if (!fieldGet.isInteger(closedWorld)) return info.newUnboundRange();

[Johnni Winther, 2017/02/02 08:33:36]

Why is this case removed?

-    if (!fieldGet.receiver.isIndexablePrimitive(closedWorld)) {
-      return visitInstruction(fieldGet);
-    }
-    assert(fieldGet.element == backendHelpers.jsIndexableLength);
-    PositiveValue value = info.newPositiveValue(fieldGet);
+    return visitInstruction(fieldGet);
+  }
+
+  Range visitGetLength(HGetLength node) {
+    PositiveValue value = info.newPositiveValue(node);
     // We know this range is above zero. To simplify the analysis, we
     // put the zero value as the lower bound of this range. This
     // allows to easily remove the second bound check in the following
     // expression: a[1] + a[0].
     return info.newNormalizedRange(info.intZero, value);
   }
 
   Range visitBoundsCheck(HBoundsCheck check) {
     // Save the next instruction, in case the check gets removed.
     HInstruction next = check.next;
@@ skipping 388 matching lines @@
   }
 
   Range handleBinaryOperation(HBinaryArithmetic instruction) {
     Range leftRange = visit(instruction.left);
     Range rightRange = visit(instruction.right);
     if (leftRange == null || rightRange == null) return null;
     BinaryOperation operation = instruction.operation(info.constantSystem);
     return operation.apply(leftRange, rightRange);
   }
 }