dart2js cps: Handle unsafe .length calls in bounds check elimination.

pkg/compiler/lib/src/cps_ir/bounds_checker.dart

 // Copyright (c) 2015, 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.
 
 library dart2js.cps_ir.bounds_checker;
 
 import 'cps_ir_nodes.dart';
 import 'optimizers.dart' show Pass;
 import 'octagon.dart';
 import '../constants/values.dart';
@@ skipping 596 matching lines @@
         effectNumberAt[cont] = NEW_EFFECT;
       }
       // TODO(asgerf): Compute join for parameters to increase precision?
     }
   }
 
   // ---------------- PRIMITIVES --------------------
 
   @override
   void visitInvokeMethod(InvokeMethod node) {
+    if (node.selector.isGetter && node.selector.name == 'length') {
+      // If the receiver type is not known to be indexable, the length call
+      // was not rewritten to GetLength.  But if we can prove that the call only
+      // succeeds for indexables, we can trust that it returns the length.
+      TypeMask successType =
+          types.receiverTypeFor(node.selector, node.dartReceiver.type);
+      if (types.isDefinitelyIndexable(successType)) {
+        valueOf[node] = getLength(node.dartReceiver, currentEffectNumber);
+      }
+    }
     // TODO(asgerf): What we really need is a "changes length" side effect flag.
     if (world
         .getSideEffectsOfSelector(node.selector, node.mask)
         .changesIndex()) {
       currentEffectNumber = makeNewEffect();
     }
   }
 
   @override
   void visitInvokeStatic(InvokeStatic node) {
@@ skipping 104 matching lines @@
     }
     return node.body;
   }
 }
 
 /// Lattice representing the known (weak) monotonicity of a loop variable.
 ///
 /// The lattice bottom is represented by `null` and represents the case where
 /// the loop variable never changes value during the loop.
 enum Monotonicity { NotMonotone, Increasing, Decreasing, }